All pages were already initialized and set to PageReserved() with a
refcount of 1 by MM init code.

In fact, by using __init_single_page(), we will be setting the refcount to
1 just to freeze it again immediately afterwards.

So drop the __init_single_page() and use __ClearPageReserved() instead.
Adjust the comments to highlight that we are dealing with an open-coded
prep_compound_page() variant.

Further, as we can now safely iterate over all pages in a folio, let's
avoid the page-pfn dance and just iterate the pages directly.

Note that the current code was likely problematic, but we never ran into
it: prep_compound_tail() would have been called with an offset that might
exceed a memory section, and prep_compound_tail() would have simply
added that offset to the page pointer -- which would not have done the
right thing on sparsemem without vmemmap.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/hugetlb.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d12a9d5146af4..ae82a845b14ad 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3235,17 +3235,14 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
 					unsigned long start_page_number,
 					unsigned long end_page_number)
 {
-	enum zone_type zone = zone_idx(folio_zone(folio));
-	int nid = folio_nid(folio);
-	unsigned long head_pfn = folio_pfn(folio);
-	unsigned long pfn, end_pfn = head_pfn + end_page_number;
+	struct page *head_page = folio_page(folio, 0);
+	struct page *page = folio_page(folio, start_page_number);
+	unsigned long i;
 	int ret;
 
-	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
-		struct page *page = pfn_to_page(pfn);
-
-		__init_single_page(page, pfn, zone, nid);
-		prep_compound_tail((struct page *)folio, pfn - head_pfn);
+	for (i = start_page_number; i < end_page_number; i++, page++) {
+		__ClearPageReserved(page);
+		prep_compound_tail(head_page, i);
 		ret = page_ref_freeze(page, 1);
 		VM_BUG_ON(!ret);
 	}
@@ -3257,12 +3254,14 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
 {
 	int ret;
 
-	/* Prepare folio head */
+	/*
+	 * This is an open-coded prep_compound_page() whereby we avoid
+	 * walking pages twice by preparing+freezing them in the same go.
+	 */
 	__folio_clear_reserved(folio);
 	__folio_set_head(folio);
 	ret = folio_ref_freeze(folio, 1);
 	VM_BUG_ON(!ret);
-	/* Initialize the necessary tail struct pages */
 	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
 	prep_compound_head((struct page *)folio, huge_page_order(h));
 }
-- 
2.50.1

On 8/21/25 23:06, David Hildenbrand wrote:

> All pages were already initialized and set to PageReserved() with a
> refcount of 1 by MM init code.

Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
initialize struct pages?

> In fact, by using __init_single_page(), we will be setting the refcount to
> 1 just to freeze it again immediately afterwards.
>
> So drop the __init_single_page() and use __ClearPageReserved() instead.
> Adjust the comments to highlight that we are dealing with an open-coded
> prep_compound_page() variant.
>
> Further, as we can now safely iterate over all pages in a folio, let's
> avoid the page-pfn dance and just iterate the pages directly.
>
> Note that the current code was likely problematic, but we never ran into
> it: prep_compound_tail() would have been called with an offset that might
> exceed a memory section, and prep_compound_tail() would have simply
> added that offset to the page pointer -- which would not have done the
> right thing on sparsemem without vmemmap.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/hugetlb.c | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index d12a9d5146af4..ae82a845b14ad 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3235,17 +3235,14 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
>  					unsigned long start_page_number,
>  					unsigned long end_page_number)
>  {
> -	enum zone_type zone = zone_idx(folio_zone(folio));
> -	int nid = folio_nid(folio);
> -	unsigned long head_pfn = folio_pfn(folio);
> -	unsigned long pfn, end_pfn = head_pfn + end_page_number;
> +	struct page *head_page = folio_page(folio, 0);
> +	struct page *page = folio_page(folio, start_page_number);
> +	unsigned long i;
>  	int ret;
>  
> -	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
> -		struct page *page = pfn_to_page(pfn);
> -
> -		__init_single_page(page, pfn, zone, nid);
> -		prep_compound_tail((struct page *)folio, pfn - head_pfn);
> +	for (i = start_page_number; i < end_page_number; i++, page++) {
> +		__ClearPageReserved(page);
> +		prep_compound_tail(head_page, i);
>  		ret = page_ref_freeze(page, 1);
>  		VM_BUG_ON(!ret);
>  	}
> @@ -3257,12 +3254,14 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
>  {
>  	int ret;
>  
> -	/* Prepare folio head */
> +	/*
> +	 * This is an open-coded prep_compound_page() whereby we avoid
> +	 * walking pages twice by preparing+freezing them in the same go.
> +	 */
>  	__folio_clear_reserved(folio);
>  	__folio_set_head(folio);
>  	ret = folio_ref_freeze(folio, 1);
>  	VM_BUG_ON(!ret);
> -	/* Initialize the necessary tail struct pages */
>  	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
>  	prep_compound_head((struct page *)folio, huge_page_order(h));
>  }

--Mika

On 22.08.25 06:09, Mika Penttilä wrote:
> 
> On 8/21/25 23:06, David Hildenbrand wrote:
> 
>> All pages were already initialized and set to PageReserved() with a
>> refcount of 1 by MM init code.
> 
> Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
> initialize struct pages?

Excellent point, I did not know about that one.

Spotting that we don't do the same for the head page made me assume that 
it's just a misuse of __init_single_page().

But the nasty thing is that we use memblock_reserved_mark_noinit() to 
only mark the tail pages ...

Let me revert back to __init_single_page() and add a big fat comment why 
this is required.

Thanks!

-- 
Cheers

David / dhildenb

On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
> On 22.08.25 06:09, Mika Penttilä wrote:
> > 
> > On 8/21/25 23:06, David Hildenbrand wrote:
> > 
> > > All pages were already initialized and set to PageReserved() with a
> > > refcount of 1 by MM init code.
> > 
> > Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
> > initialize struct pages?
> 
> Excellent point, I did not know about that one.
> 
> Spotting that we don't do the same for the head page made me assume that
> it's just a misuse of __init_single_page().
> 
> But the nasty thing is that we use memblock_reserved_mark_noinit() to only
> mark the tail pages ...

And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
disabled struct pages are initialized regardless of
memblock_reserved_mark_noinit().

I think this patch should go in before your updates:

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 753f99b4c718..1c51788339a5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3230,6 +3230,22 @@ int __alloc_bootmem_huge_page(struct hstate *h, int nid)
 	return 1;
 }
 
+/*
+ * Tail pages in a huge folio allocated from memblock are marked as 'noinit',
+ * which means that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled their
+ * struct page won't be initialized
+ */
+#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
+static void __init hugetlb_init_tail_page(struct page *page, unsigned long pfn,
+					enum zone_type zone, int nid)
+{
+	__init_single_page(page, pfn, zone, nid);
+}
+#else
+static inline void hugetlb_init_tail_page(struct page *page, unsigned long pfn,
+					enum zone_type zone, int nid) {}
+#endif
+
 /* Initialize [start_page:end_page_number] tail struct pages of a hugepage */
 static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
 					unsigned long start_page_number,
@@ -3244,7 +3260,7 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
 	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
 		struct page *page = pfn_to_page(pfn);
 
-		__init_single_page(page, pfn, zone, nid);
+		hugetlb_init_tail_page(page, pfn, zone, nid);
 		prep_compound_tail((struct page *)folio, pfn - head_pfn);
 		ret = page_ref_freeze(page, 1);
 		VM_BUG_ON(!ret);
 
> Let me revert back to __init_single_page() and add a big fat comment why
> this is required.
> 
> Thanks!

-- 
Sincerely yours,
Mike.

On 23.08.25 10:59, Mike Rapoport wrote:
> On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
>> On 22.08.25 06:09, Mika Penttilä wrote:
>>>
>>> On 8/21/25 23:06, David Hildenbrand wrote:
>>>
>>>> All pages were already initialized and set to PageReserved() with a
>>>> refcount of 1 by MM init code.
>>>
>>> Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
>>> initialize struct pages?
>>
>> Excellent point, I did not know about that one.
>>
>> Spotting that we don't do the same for the head page made me assume that
>> it's just a misuse of __init_single_page().
>>
>> But the nasty thing is that we use memblock_reserved_mark_noinit() to only
>> mark the tail pages ...
> 
> And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
> disabled struct pages are initialized regardless of
> memblock_reserved_mark_noinit().
> 
> I think this patch should go in before your updates:

Shouldn't we fix this in memblock code?

Hacking around that in the memblock_reserved_mark_noinit() user sound 
wrong -- and nothing in the doc of memblock_reserved_mark_noinit() 
spells that behavior out.

-- 
Cheers

David / dhildenb

On Mon, Aug 25, 2025 at 02:48:58PM +0200, David Hildenbrand wrote:
> On 23.08.25 10:59, Mike Rapoport wrote:
> > On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
> > > On 22.08.25 06:09, Mika Penttilä wrote:
> > > > 
> > > > On 8/21/25 23:06, David Hildenbrand wrote:
> > > > 
> > > > > All pages were already initialized and set to PageReserved() with a
> > > > > refcount of 1 by MM init code.
> > > > 
> > > > Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
> > > > initialize struct pages?
> > > 
> > > Excellent point, I did not know about that one.
> > > 
> > > Spotting that we don't do the same for the head page made me assume that
> > > it's just a misuse of __init_single_page().
> > > 
> > > But the nasty thing is that we use memblock_reserved_mark_noinit() to only
> > > mark the tail pages ...
> > 
> > And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
> > disabled struct pages are initialized regardless of
> > memblock_reserved_mark_noinit().
> > 
> > I think this patch should go in before your updates:
> 
> Shouldn't we fix this in memblock code?
> 
> Hacking around that in the memblock_reserved_mark_noinit() user sound wrong
> -- and nothing in the doc of memblock_reserved_mark_noinit() spells that
> behavior out.

We can surely update the docs, but unfortunately I don't see how to avoid
hacking around it in hugetlb. 
Since it's used to optimise HVO even further to the point hugetlb open
codes memmap initialization, I think it's fair that it should deal with all
possible configurations.
 
> -- 
> Cheers
> 
> David / dhildenb
> 
> 

-- 
Sincerely yours,
Mike.

On 25.08.25 16:32, Mike Rapoport wrote:
> On Mon, Aug 25, 2025 at 02:48:58PM +0200, David Hildenbrand wrote:
>> On 23.08.25 10:59, Mike Rapoport wrote:
>>> On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
>>>> On 22.08.25 06:09, Mika Penttilä wrote:
>>>>>
>>>>> On 8/21/25 23:06, David Hildenbrand wrote:
>>>>>
>>>>>> All pages were already initialized and set to PageReserved() with a
>>>>>> refcount of 1 by MM init code.
>>>>>
>>>>> Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
>>>>> initialize struct pages?
>>>>
>>>> Excellent point, I did not know about that one.
>>>>
>>>> Spotting that we don't do the same for the head page made me assume that
>>>> it's just a misuse of __init_single_page().
>>>>
>>>> But the nasty thing is that we use memblock_reserved_mark_noinit() to only
>>>> mark the tail pages ...
>>>
>>> And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
>>> disabled struct pages are initialized regardless of
>>> memblock_reserved_mark_noinit().
>>>
>>> I think this patch should go in before your updates:
>>
>> Shouldn't we fix this in memblock code?
>>
>> Hacking around that in the memblock_reserved_mark_noinit() user sound wrong
>> -- and nothing in the doc of memblock_reserved_mark_noinit() spells that
>> behavior out.
> 
> We can surely update the docs, but unfortunately I don't see how to avoid
> hacking around it in hugetlb.
> Since it's used to optimise HVO even further to the point hugetlb open
> codes memmap initialization, I think it's fair that it should deal with all
> possible configurations.

Remind me, why can't we support memblock_reserved_mark_noinit() when 
CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled?

-- 
Cheers

David / dhildenb

On Mon, Aug 25, 2025 at 04:38:03PM +0200, David Hildenbrand wrote:
> On 25.08.25 16:32, Mike Rapoport wrote:
> > On Mon, Aug 25, 2025 at 02:48:58PM +0200, David Hildenbrand wrote:
> > > On 23.08.25 10:59, Mike Rapoport wrote:
> > > > On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
> > > > > On 22.08.25 06:09, Mika Penttilä wrote:
> > > > > > 
> > > > > > On 8/21/25 23:06, David Hildenbrand wrote:
> > > > > > 
> > > > > > > All pages were already initialized and set to PageReserved() with a
> > > > > > > refcount of 1 by MM init code.
> > > > > > 
> > > > > > Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
> > > > > > initialize struct pages?
> > > > > 
> > > > > Excellent point, I did not know about that one.
> > > > > 
> > > > > Spotting that we don't do the same for the head page made me assume that
> > > > > it's just a misuse of __init_single_page().
> > > > > 
> > > > > But the nasty thing is that we use memblock_reserved_mark_noinit() to only
> > > > > mark the tail pages ...
> > > > 
> > > > And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
> > > > disabled struct pages are initialized regardless of
> > > > memblock_reserved_mark_noinit().
> > > > 
> > > > I think this patch should go in before your updates:
> > > 
> > > Shouldn't we fix this in memblock code?
> > > 
> > > Hacking around that in the memblock_reserved_mark_noinit() user sound wrong
> > > -- and nothing in the doc of memblock_reserved_mark_noinit() spells that
> > > behavior out.
> > 
> > We can surely update the docs, but unfortunately I don't see how to avoid
> > hacking around it in hugetlb.
> > Since it's used to optimise HVO even further to the point hugetlb open
> > codes memmap initialization, I think it's fair that it should deal with all
> > possible configurations.
> 
> Remind me, why can't we support memblock_reserved_mark_noinit() when
> CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled?

When CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled we initialize the entire
memmap early (setup_arch()->free_area_init()), and we may have a bunch of
memblock_reserved_mark_noinit() afterwards
 
> -- 
> Cheers
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.

On 25.08.25 16:59, Mike Rapoport wrote:
> On Mon, Aug 25, 2025 at 04:38:03PM +0200, David Hildenbrand wrote:
>> On 25.08.25 16:32, Mike Rapoport wrote:
>>> On Mon, Aug 25, 2025 at 02:48:58PM +0200, David Hildenbrand wrote:
>>>> On 23.08.25 10:59, Mike Rapoport wrote:
>>>>> On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
>>>>>> On 22.08.25 06:09, Mika Penttilä wrote:
>>>>>>>
>>>>>>> On 8/21/25 23:06, David Hildenbrand wrote:
>>>>>>>
>>>>>>>> All pages were already initialized and set to PageReserved() with a
>>>>>>>> refcount of 1 by MM init code.
>>>>>>>
>>>>>>> Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
>>>>>>> initialize struct pages?
>>>>>>
>>>>>> Excellent point, I did not know about that one.
>>>>>>
>>>>>> Spotting that we don't do the same for the head page made me assume that
>>>>>> it's just a misuse of __init_single_page().
>>>>>>
>>>>>> But the nasty thing is that we use memblock_reserved_mark_noinit() to only
>>>>>> mark the tail pages ...
>>>>>
>>>>> And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
>>>>> disabled struct pages are initialized regardless of
>>>>> memblock_reserved_mark_noinit().
>>>>>
>>>>> I think this patch should go in before your updates:
>>>>
>>>> Shouldn't we fix this in memblock code?
>>>>
>>>> Hacking around that in the memblock_reserved_mark_noinit() user sound wrong
>>>> -- and nothing in the doc of memblock_reserved_mark_noinit() spells that
>>>> behavior out.
>>>
>>> We can surely update the docs, but unfortunately I don't see how to avoid
>>> hacking around it in hugetlb.
>>> Since it's used to optimise HVO even further to the point hugetlb open
>>> codes memmap initialization, I think it's fair that it should deal with all
>>> possible configurations.
>>
>> Remind me, why can't we support memblock_reserved_mark_noinit() when
>> CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled?
> 
> When CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled we initialize the entire
> memmap early (setup_arch()->free_area_init()), and we may have a bunch of
> memblock_reserved_mark_noinit() afterwards

Oh, you mean that we get effective memblock modifications after already
initializing the memmap.

That sounds ... interesting :)

So yeah, we have to document this for memblock_reserved_mark_noinit().

Is it also a problem for kexec_handover?

We should do something like:

diff --git a/mm/memblock.c b/mm/memblock.c
index 154f1d73b61f2..ed4c563d72c32 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1091,13 +1091,16 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
  
  /**
   * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
- * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
- * for this region.
+ * MEMBLOCK_RSRV_NOINIT which allows for the "struct pages" corresponding
+ * to this region not getting initialized, because the caller will take
+ * care of it.
   * @base: the base phys addr of the region
   * @size: the size of the region
   *
- * struct pages will not be initialized for reserved memory regions marked with
- * %MEMBLOCK_RSRV_NOINIT.
+ * "struct pages" will not be initialized for reserved memory regions marked
+ * with %MEMBLOCK_RSRV_NOINIT if this function is called before initialization
+ * code runs. Without CONFIG_DEFERRED_STRUCT_PAGE_INIT, it is more likely
+ * that this function is not effective.
   *
   * Return: 0 on success, -errno on failure.
   */


Optimizing the hugetlb code could be done, but I am not sure how high
the priority is (nobody complained so far about the double init).

-- 
Cheers

David / dhildenb

On Mon, Aug 25, 2025 at 05:42:33PM +0200, David Hildenbrand wrote:
> On 25.08.25 16:59, Mike Rapoport wrote:
> > On Mon, Aug 25, 2025 at 04:38:03PM +0200, David Hildenbrand wrote:
> > > On 25.08.25 16:32, Mike Rapoport wrote:
> > > > On Mon, Aug 25, 2025 at 02:48:58PM +0200, David Hildenbrand wrote:
> > > > > On 23.08.25 10:59, Mike Rapoport wrote:
> > > > > > On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
> > > > > > > On 22.08.25 06:09, Mika Penttilä wrote:
> > > > > > > > 
> > > > > > > > On 8/21/25 23:06, David Hildenbrand wrote:
> > > > > > > > 
> > > > > > > > > All pages were already initialized and set to PageReserved() with a
> > > > > > > > > refcount of 1 by MM init code.
> > > > > > > > 
> > > > > > > > Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
> > > > > > > > initialize struct pages?
> > > > > > > 
> > > > > > > Excellent point, I did not know about that one.
> > > > > > > 
> > > > > > > Spotting that we don't do the same for the head page made me assume that
> > > > > > > it's just a misuse of __init_single_page().
> > > > > > > 
> > > > > > > But the nasty thing is that we use memblock_reserved_mark_noinit() to only
> > > > > > > mark the tail pages ...
> > > > > > 
> > > > > > And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
> > > > > > disabled struct pages are initialized regardless of
> > > > > > memblock_reserved_mark_noinit().
> > > > > > 
> > > > > > I think this patch should go in before your updates:
> > > > > 
> > > > > Shouldn't we fix this in memblock code?
> > > > > 
> > > > > Hacking around that in the memblock_reserved_mark_noinit() user sound wrong
> > > > > -- and nothing in the doc of memblock_reserved_mark_noinit() spells that
> > > > > behavior out.
> > > > 
> > > > We can surely update the docs, but unfortunately I don't see how to avoid
> > > > hacking around it in hugetlb.
> > > > Since it's used to optimise HVO even further to the point hugetlb open
> > > > codes memmap initialization, I think it's fair that it should deal with all
> > > > possible configurations.
> > > 
> > > Remind me, why can't we support memblock_reserved_mark_noinit() when
> > > CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled?
> > 
> > When CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled we initialize the entire
> > memmap early (setup_arch()->free_area_init()), and we may have a bunch of
> > memblock_reserved_mark_noinit() afterwards
> 
> Oh, you mean that we get effective memblock modifications after already
> initializing the memmap.
> 
> That sounds ... interesting :)

It's memmap, not the free lists. Without deferred init, memblock is active
for a while after memmap initialized and before the memory goes to the free
lists.
 
> So yeah, we have to document this for memblock_reserved_mark_noinit().
> 
> Is it also a problem for kexec_handover?

With KHO it's also interesting, but it does not support deferred struct
page init for now :)
 
> We should do something like:
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 154f1d73b61f2..ed4c563d72c32 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1091,13 +1091,16 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>  /**
>   * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
> - * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
> - * for this region.
> + * MEMBLOCK_RSRV_NOINIT which allows for the "struct pages" corresponding
> + * to this region not getting initialized, because the caller will take
> + * care of it.
>   * @base: the base phys addr of the region
>   * @size: the size of the region
>   *
> - * struct pages will not be initialized for reserved memory regions marked with
> - * %MEMBLOCK_RSRV_NOINIT.
> + * "struct pages" will not be initialized for reserved memory regions marked
> + * with %MEMBLOCK_RSRV_NOINIT if this function is called before initialization
> + * code runs. Without CONFIG_DEFERRED_STRUCT_PAGE_INIT, it is more likely
> + * that this function is not effective.
>   *
>   * Return: 0 on success, -errno on failure.
>   */

I have a different version :)
 
diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index b96746376e17..d20d091c6343 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h
@@ -40,8 +40,9 @@ extern unsigned long long max_possible_pfn;
  * via a driver, and never indicated in the firmware-provided memory map as
  * system RAM. This corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED in the
  * kernel resource tree.
- * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages are
- * not initialized (only for reserved regions).
+ * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages don't have
+ * PG_Reserved set and are completely not initialized when
+ * %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled (only for reserved regions).
  * @MEMBLOCK_RSRV_KERN: memory region that is reserved for kernel use,
  * either explictitly with memblock_reserve_kern() or via memblock
  * allocation APIs. All memblock allocations set this flag.
diff --git a/mm/memblock.c b/mm/memblock.c
index 154f1d73b61f..02de5ffb085b 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1091,13 +1091,15 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
 
 /**
  * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
- * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
- * for this region.
+ * MEMBLOCK_RSRV_NOINIT
+ *
  * @base: the base phys addr of the region
  * @size: the size of the region
  *
- * struct pages will not be initialized for reserved memory regions marked with
- * %MEMBLOCK_RSRV_NOINIT.
+ * The struct pages for the reserved regions marked %MEMBLOCK_RSRV_NOINIT will
+ * not have %PG_Reserved flag set.
+ * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, setting this flags also
+ * completly bypasses the initialization of struct pages for this region.
  *
  * Return: 0 on success, -errno on failure.
  */
 
> Optimizing the hugetlb code could be done, but I am not sure how high
> the priority is (nobody complained so far about the double init).
> 
> -- 
> Cheers
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.

>   
>> We should do something like:
>>
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index 154f1d73b61f2..ed4c563d72c32 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -1091,13 +1091,16 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>>   /**
>>    * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
>> - * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
>> - * for this region.
>> + * MEMBLOCK_RSRV_NOINIT which allows for the "struct pages" corresponding
>> + * to this region not getting initialized, because the caller will take
>> + * care of it.
>>    * @base: the base phys addr of the region
>>    * @size: the size of the region
>>    *
>> - * struct pages will not be initialized for reserved memory regions marked with
>> - * %MEMBLOCK_RSRV_NOINIT.
>> + * "struct pages" will not be initialized for reserved memory regions marked
>> + * with %MEMBLOCK_RSRV_NOINIT if this function is called before initialization
>> + * code runs. Without CONFIG_DEFERRED_STRUCT_PAGE_INIT, it is more likely
>> + * that this function is not effective.
>>    *
>>    * Return: 0 on success, -errno on failure.
>>    */
> 
> I have a different version :)
>   
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index b96746376e17..d20d091c6343 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -40,8 +40,9 @@ extern unsigned long long max_possible_pfn;
>    * via a driver, and never indicated in the firmware-provided memory map as
>    * system RAM. This corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED in the
>    * kernel resource tree.
> - * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages are
> - * not initialized (only for reserved regions).
> + * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages don't have
> + * PG_Reserved set and are completely not initialized when
> + * %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled (only for reserved regions).
>    * @MEMBLOCK_RSRV_KERN: memory region that is reserved for kernel use,
>    * either explictitly with memblock_reserve_kern() or via memblock
>    * allocation APIs. All memblock allocations set this flag.
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 154f1d73b61f..02de5ffb085b 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1091,13 +1091,15 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>   
>   /**
>    * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
> - * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
> - * for this region.
> + * MEMBLOCK_RSRV_NOINIT
> + *
>    * @base: the base phys addr of the region
>    * @size: the size of the region
>    *
> - * struct pages will not be initialized for reserved memory regions marked with
> - * %MEMBLOCK_RSRV_NOINIT.
> + * The struct pages for the reserved regions marked %MEMBLOCK_RSRV_NOINIT will
> + * not have %PG_Reserved flag set.
> + * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, setting this flags also
> + * completly bypasses the initialization of struct pages for this region.

s/completly/completely.

I don't quite understand the interaction with PG_Reserved and why 
anybody using this function should care.

So maybe you can rephrase in a way that is easier to digest, and rather 
focuses on what callers of this function are supposed to do vs. have the 
liberty of not doing?

-- 
Cheers

David / dhildenb

On Mon, Aug 25, 2025 at 06:23:48PM +0200, David Hildenbrand wrote:
> 
> I don't quite understand the interaction with PG_Reserved and why anybody
> using this function should care.
> 
> So maybe you can rephrase in a way that is easier to digest, and rather
> focuses on what callers of this function are supposed to do vs. have the
> liberty of not doing?

How about
 
diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index b96746376e17..fcda8481de9a 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h
@@ -40,8 +40,9 @@ extern unsigned long long max_possible_pfn;
  * via a driver, and never indicated in the firmware-provided memory map as
  * system RAM. This corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED in the
  * kernel resource tree.
- * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages are
- * not initialized (only for reserved regions).
+ * @MEMBLOCK_RSRV_NOINIT: reserved memory region for which struct pages are not
+ * fully initialized. Users of this flag are responsible to properly initialize
+ * struct pages of this region
  * @MEMBLOCK_RSRV_KERN: memory region that is reserved for kernel use,
  * either explictitly with memblock_reserve_kern() or via memblock
  * allocation APIs. All memblock allocations set this flag.
diff --git a/mm/memblock.c b/mm/memblock.c
index 154f1d73b61f..46b411fb3630 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1091,13 +1091,20 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
 
 /**
  * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
- * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
- * for this region.
+ * MEMBLOCK_RSRV_NOINIT
+ *
  * @base: the base phys addr of the region
  * @size: the size of the region
  *
- * struct pages will not be initialized for reserved memory regions marked with
- * %MEMBLOCK_RSRV_NOINIT.
+ * The struct pages for the reserved regions marked %MEMBLOCK_RSRV_NOINIT will
+ * not be fully initialized to allow the caller optimize their initialization.
+ *
+ * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, setting this flag
+ * completely bypasses the initialization of struct pages for such region.
+ *
+ * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled, struct pages in this
+ * region will be initialized with default values but won't be marked as
+ * reserved.
  *
  * Return: 0 on success, -errno on failure.
  */

> -- 
> Cheers
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.

On 25.08.25 18:58, Mike Rapoport wrote:
> On Mon, Aug 25, 2025 at 06:23:48PM +0200, David Hildenbrand wrote:
>>
>> I don't quite understand the interaction with PG_Reserved and why anybody
>> using this function should care.
>>
>> So maybe you can rephrase in a way that is easier to digest, and rather
>> focuses on what callers of this function are supposed to do vs. have the
>> liberty of not doing?
> 
> How about
>   
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index b96746376e17..fcda8481de9a 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -40,8 +40,9 @@ extern unsigned long long max_possible_pfn;
>    * via a driver, and never indicated in the firmware-provided memory map as
>    * system RAM. This corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED in the
>    * kernel resource tree.
> - * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages are
> - * not initialized (only for reserved regions).
> + * @MEMBLOCK_RSRV_NOINIT: reserved memory region for which struct pages are not
> + * fully initialized. Users of this flag are responsible to properly initialize
> + * struct pages of this region
>    * @MEMBLOCK_RSRV_KERN: memory region that is reserved for kernel use,
>    * either explictitly with memblock_reserve_kern() or via memblock
>    * allocation APIs. All memblock allocations set this flag.
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 154f1d73b61f..46b411fb3630 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1091,13 +1091,20 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>   
>   /**
>    * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
> - * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
> - * for this region.
> + * MEMBLOCK_RSRV_NOINIT
> + *
>    * @base: the base phys addr of the region
>    * @size: the size of the region
>    *
> - * struct pages will not be initialized for reserved memory regions marked with
> - * %MEMBLOCK_RSRV_NOINIT.
> + * The struct pages for the reserved regions marked %MEMBLOCK_RSRV_NOINIT will
> + * not be fully initialized to allow the caller optimize their initialization.
> + *
> + * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, setting this flag
> + * completely bypasses the initialization of struct pages for such region.
> + *
> + * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled, struct pages in this
> + * region will be initialized with default values but won't be marked as
> + * reserved.

Sounds good.

I am surprised regarding "reserved", but I guess that's because we don't 
end up calling "reserve_bootmem_region()" on these regions in 
memmap_init_reserved_pages().


-- 
Cheers

David / dhildenb