Marking PMDs that map a "normal" refcounted folios as special is
against our rules documented for vm_normal_page(): normal (refcounted)
folios shall never have the page table mapping marked as special.

Fortunately, there are not that many pmd_special() check that can be
mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
would get this wrong right now are rather harmless: e.g., none so far
bases decisions whether to grab a folio reference on that decision.

Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
implications as it seems.

Getting this right will get more important as we use
folio_normal_page_pmd() in more places.

Fix it by teaching insert_pfn_pmd() to properly handle folios and
pfns -- moving refcount/mapcount/etc handling in there, renaming it to
insert_pmd(), and distinguishing between both cases using a new simple
"struct folio_or_pfn" structure.

Use folio_mk_pmd() to create a pmd for a folio cleanly.

Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/huge_memory.c | 59 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 40 insertions(+), 19 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 49b98082c5401..d1e3e253c714a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1372,9 +1372,17 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 	return __do_huge_pmd_anonymous_page(vmf);
 }
 
-static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
-		pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write,
-		pgtable_t pgtable)
+struct folio_or_pfn {
+	union {
+		struct folio *folio;
+		pfn_t pfn;
+	};
+	bool is_folio;
+};
+
+static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
+		pmd_t *pmd, struct folio_or_pfn fop, pgprot_t prot,
+		bool write, pgtable_t pgtable)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	pmd_t entry;
@@ -1382,8 +1390,11 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
 	lockdep_assert_held(pmd_lockptr(mm, pmd));
 
 	if (!pmd_none(*pmd)) {
+		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
+					  pfn_t_to_pfn(fop.pfn);
+
 		if (write) {
-			if (pmd_pfn(*pmd) != pfn_t_to_pfn(pfn)) {
+			if (pmd_pfn(*pmd) != pfn) {
 				WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
 				return -EEXIST;
 			}
@@ -1396,11 +1407,20 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
 		return -EEXIST;
 	}
 
-	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
-	if (pfn_t_devmap(pfn))
-		entry = pmd_mkdevmap(entry);
-	else
-		entry = pmd_mkspecial(entry);
+	if (fop.is_folio) {
+		entry = folio_mk_pmd(fop.folio, vma->vm_page_prot);
+
+		folio_get(fop.folio);
+		folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma);
+		add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
+	} else {
+		entry = pmd_mkhuge(pfn_t_pmd(fop.pfn, prot));
+
+		if (pfn_t_devmap(fop.pfn))
+			entry = pmd_mkdevmap(entry);
+		else
+			entry = pmd_mkspecial(entry);
+	}
 	if (write) {
 		entry = pmd_mkyoung(pmd_mkdirty(entry));
 		entry = maybe_pmd_mkwrite(entry, vma);
@@ -1431,6 +1451,9 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
 	unsigned long addr = vmf->address & PMD_MASK;
 	struct vm_area_struct *vma = vmf->vma;
 	pgprot_t pgprot = vma->vm_page_prot;
+	struct folio_or_pfn fop = {
+		.pfn = pfn,
+	};
 	pgtable_t pgtable = NULL;
 	spinlock_t *ptl;
 	int error;
@@ -1458,8 +1481,8 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
 	pfnmap_setup_cachemode_pfn(pfn_t_to_pfn(pfn), &pgprot);
 
 	ptl = pmd_lock(vma->vm_mm, vmf->pmd);
-	error = insert_pfn_pmd(vma, addr, vmf->pmd, pfn, pgprot, write,
-			pgtable);
+	error = insert_pmd(vma, addr, vmf->pmd, fop, pgprot, write,
+			   pgtable);
 	spin_unlock(ptl);
 	if (error && pgtable)
 		pte_free(vma->vm_mm, pgtable);
@@ -1474,6 +1497,10 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio,
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long addr = vmf->address & PMD_MASK;
 	struct mm_struct *mm = vma->vm_mm;
+	struct folio_or_pfn fop = {
+		.folio = folio,
+		.is_folio = true,
+	};
 	spinlock_t *ptl;
 	pgtable_t pgtable = NULL;
 	int error;
@@ -1491,14 +1518,8 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio,
 	}
 
 	ptl = pmd_lock(mm, vmf->pmd);
-	if (pmd_none(*vmf->pmd)) {
-		folio_get(folio);
-		folio_add_file_rmap_pmd(folio, &folio->page, vma);
-		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR);
-	}
-	error = insert_pfn_pmd(vma, addr, vmf->pmd,
-			pfn_to_pfn_t(folio_pfn(folio)), vma->vm_page_prot,
-			write, pgtable);
+	error = insert_pmd(vma, addr, vmf->pmd, fop, vma->vm_page_prot,
+			   write, pgtable);
 	spin_unlock(ptl);
 	if (error && pgtable)
 		pte_free(mm, pgtable);
-- 
2.49.0

On Fri, Jun 13, 2025 at 11:27:01AM +0200, David Hildenbrand wrote:
> Marking PMDs that map a "normal" refcounted folios as special is
> against our rules documented for vm_normal_page(): normal (refcounted)
> folios shall never have the page table mapping marked as special.
> 
> Fortunately, there are not that many pmd_special() check that can be
> mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
> would get this wrong right now are rather harmless: e.g., none so far
> bases decisions whether to grab a folio reference on that decision.
> 
> Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
> implications as it seems.
> 
> Getting this right will get more important as we use
> folio_normal_page_pmd() in more places.
> 
> Fix it by teaching insert_pfn_pmd() to properly handle folios and
> pfns -- moving refcount/mapcount/etc handling in there, renaming it to
> insert_pmd(), and distinguishing between both cases using a new simple
> "struct folio_or_pfn" structure.
> 
> Use folio_mk_pmd() to create a pmd for a folio cleanly.
> 
> Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Tested-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Altough we have it quite well explained here in the changelog, maybe
having a little comment in insert_pmd() noting why pmds mapping normal
folios cannot be marked special would be nice.

But just saying :-)

Reviewed-by: Oscar salvador <osalvador@suse.de>

 

-- 
Oscar Salvador
SUSE Labs

On 13.06.25 15:49, Oscar Salvador wrote:
> On Fri, Jun 13, 2025 at 11:27:01AM +0200, David Hildenbrand wrote:
>> Marking PMDs that map a "normal" refcounted folios as special is
>> against our rules documented for vm_normal_page(): normal (refcounted)
>> folios shall never have the page table mapping marked as special.
>>
>> Fortunately, there are not that many pmd_special() check that can be
>> mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
>> would get this wrong right now are rather harmless: e.g., none so far
>> bases decisions whether to grab a folio reference on that decision.
>>
>> Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
>> implications as it seems.
>>
>> Getting this right will get more important as we use
>> folio_normal_page_pmd() in more places.
>>
>> Fix it by teaching insert_pfn_pmd() to properly handle folios and
>> pfns -- moving refcount/mapcount/etc handling in there, renaming it to
>> insert_pmd(), and distinguishing between both cases using a new simple
>> "struct folio_or_pfn" structure.
>>
>> Use folio_mk_pmd() to create a pmd for a folio cleanly.
>>
>> Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
>> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
>> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
>> Tested-by: Dan Williams <dan.j.williams@intel.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Altough we have it quite well explained here in the changelog, maybe
> having a little comment in insert_pmd() noting why pmds mapping normal
> folios cannot be marked special would be nice.

Well, I don't think we should be replicating that all over the place. 
The big comment above vm_normal_page() is currently our source of truth 
(which I will teak soon further).

Thanks!

-- 
Cheers,

David / dhildenb

On Fri, Jun 13, 2025 at 03:53:58PM +0200, David Hildenbrand wrote:
> On 13.06.25 15:49, Oscar Salvador wrote:
> > On Fri, Jun 13, 2025 at 11:27:01AM +0200, David Hildenbrand wrote:
> > > Marking PMDs that map a "normal" refcounted folios as special is
> > > against our rules documented for vm_normal_page(): normal (refcounted)
> > > folios shall never have the page table mapping marked as special.
> > >
> > > Fortunately, there are not that many pmd_special() check that can be
> > > mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
> > > would get this wrong right now are rather harmless: e.g., none so far
> > > bases decisions whether to grab a folio reference on that decision.
> > >
> > > Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
> > > implications as it seems.
> > >
> > > Getting this right will get more important as we use
> > > folio_normal_page_pmd() in more places.
> > >
> > > Fix it by teaching insert_pfn_pmd() to properly handle folios and
> > > pfns -- moving refcount/mapcount/etc handling in there, renaming it to
> > > insert_pmd(), and distinguishing between both cases using a new simple
> > > "struct folio_or_pfn" structure.
> > >
> > > Use folio_mk_pmd() to create a pmd for a folio cleanly.
> > >
> > > Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
> > > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > > Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> > > Tested-by: Dan Williams <dan.j.williams@intel.com>
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> >
> > Altough we have it quite well explained here in the changelog, maybe
> > having a little comment in insert_pmd() noting why pmds mapping normal
> > folios cannot be marked special would be nice.
>
> Well, I don't think we should be replicating that all over the place. The
> big comment above vm_normal_page() is currently our source of truth (which I
> will teak soon further).

Suggestion:

"Kinda self-explanatory (special means don't touch) unless you use museum piece
hardware OR IF YOU ARE XEN!"

;)

>
> Thanks!
>
> --
> Cheers,
>
> David / dhildenb
>

On 13.06.25 16:00, Lorenzo Stoakes wrote:
> On Fri, Jun 13, 2025 at 03:53:58PM +0200, David Hildenbrand wrote:
>> On 13.06.25 15:49, Oscar Salvador wrote:
>>> On Fri, Jun 13, 2025 at 11:27:01AM +0200, David Hildenbrand wrote:
>>>> Marking PMDs that map a "normal" refcounted folios as special is
>>>> against our rules documented for vm_normal_page(): normal (refcounted)
>>>> folios shall never have the page table mapping marked as special.
>>>>
>>>> Fortunately, there are not that many pmd_special() check that can be
>>>> mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
>>>> would get this wrong right now are rather harmless: e.g., none so far
>>>> bases decisions whether to grab a folio reference on that decision.
>>>>
>>>> Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
>>>> implications as it seems.
>>>>
>>>> Getting this right will get more important as we use
>>>> folio_normal_page_pmd() in more places.
>>>>
>>>> Fix it by teaching insert_pfn_pmd() to properly handle folios and
>>>> pfns -- moving refcount/mapcount/etc handling in there, renaming it to
>>>> insert_pmd(), and distinguishing between both cases using a new simple
>>>> "struct folio_or_pfn" structure.
>>>>
>>>> Use folio_mk_pmd() to create a pmd for a folio cleanly.
>>>>
>>>> Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
>>>> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
>>>> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>>> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
>>>> Tested-by: Dan Williams <dan.j.williams@intel.com>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>
>>> Altough we have it quite well explained here in the changelog, maybe
>>> having a little comment in insert_pmd() noting why pmds mapping normal
>>> folios cannot be marked special would be nice.
>>
>> Well, I don't think we should be replicating that all over the place. The
>> big comment above vm_normal_page() is currently our source of truth (which I
>> will teak soon further).
> 
> Suggestion:
> 
> "Kinda self-explanatory (special means don't touch) unless you use museum piece
> hardware OR IF YOU ARE XEN!"
> 
> ;)

I looked into the XEN stuff and it is *extremely* nasty.

No, it doesn't do a pte_mkspecial(). It updates the PTE using ...

	!!! A HYPERCALL !!!

WTF, why did we ever allow that.

It's documented to require GUP to work because ... QEMU AIO. Otherwise 
we could easily convert it to a proper PFNMAP.

-- 
Cheers,

David / dhildenb

On Fri, Jun 13, 2025 at 03:49:46PM +0200, Oscar Salvador wrote:
> Reviewed-by: Oscar salvador <osalvador@suse.de>

Fat-fingers on a Friday afternoon:

Reviewed-by: Oscar Salvador <osalvador@suse.de>
 

-- 
Oscar Salvador
SUSE Labs