1. Patchew
  2. linux
  3. View series

[PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count

Jane Chu posted 1 patch 2 weeks, 2 days ago
There is a newer version of this series
include/linux/mm_types.h |  5 +++++
mm/hugetlb.c             | 15 +++++----------
2 files changed, 10 insertions(+), 10 deletions(-)
[PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count
Posted by Jane Chu 2 weeks, 2 days ago 
commit 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
introduced ->pt_share_count dedicated to hugetlb PMD share count tracking,
but omitted fixing copy_hugetlb_page_range(), leaving the function relying on
page_count() for tracking that no longer works.

When lazy page table copy for hugetlb is disabled, that is, revert
commit bcd51a3c679d ("hugetlb: lazy page table copies in fork()")
fork()'ing with hugetlb PMD sharing quickly lockup -

[  239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s!
[  239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0
[  239.446631] Call Trace:
[  239.446633]  <TASK>
[  239.446636]  _raw_spin_lock+0x3f/0x60
[  239.446639]  copy_hugetlb_page_range+0x258/0xb50
[  239.446645]  copy_page_range+0x22b/0x2c0
[  239.446651]  dup_mmap+0x3e2/0x770
[  239.446654]  dup_mm.constprop.0+0x5e/0x230
[  239.446657]  copy_process+0xd17/0x1760
[  239.446660]  kernel_clone+0xc0/0x3e0
[  239.446661]  __do_sys_clone+0x65/0xa0
[  239.446664]  do_syscall_64+0x82/0x930
[  239.446668]  ? count_memcg_events+0xd2/0x190
[  239.446671]  ? syscall_trace_enter+0x14e/0x1f0
[  239.446676]  ? syscall_exit_work+0x118/0x150
[  239.446677]  ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0
[  239.446681]  ? clear_bhb_loop+0x30/0x80
[  239.446684]  ? clear_bhb_loop+0x30/0x80
[  239.446686]  entry_SYSCALL_64_after_hwframe+0x76/0x7e

There are two options to resolve the potential latent issue:
  1. warn against PMD sharing in copy_hugetlb_page_range(),
  2. fix it.
This patch opts for the second option.
While at it, simplify the comment, the details are not actually relevant
anymore.

Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
Signed-off-by: Jane Chu <jane.chu@oracle.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
---
 include/linux/mm_types.h |  5 +++++
 mm/hugetlb.c             | 15 +++++----------
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 08bc2442db93..a643fae8a349 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -631,6 +631,11 @@ static inline int ptdesc_pmd_pts_count(struct ptdesc *ptdesc)
 {
 	return atomic_read(&ptdesc->pt_share_count);
 }
+
+static inline bool ptdesc_pmd_is_shared(struct ptdesc *ptdesc)
+{
+	return !!ptdesc_pmd_pts_count(ptdesc);
+}
 #else
 static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc)
 {
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index eed59cfb5d21..6cfe0b43ab8f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5594,18 +5594,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 			break;
 		}
 
-		/*
-		 * If the pagetables are shared don't copy or take references.
-		 *
-		 * dst_pte == src_pte is the common case of src/dest sharing.
-		 * However, src could have 'unshared' and dst shares with
-		 * another vma. So page_count of ptep page is checked instead
-		 * to reliably determine whether pte is shared.
-		 */
-		if (page_count(virt_to_page(dst_pte)) > 1) {
+#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
+		/* If the pagetables are shared, there is nothing to do */
+		if (ptdesc_pmd_is_shared(virt_to_ptdesc(dst_pte))) {
 			addr |= last_addr_mask;
 			continue;
 		}
+#endif
 
 		dst_ptl = huge_pte_lock(h, dst, dst_pte);
 		src_ptl = huge_pte_lockptr(h, src, src_pte);
@@ -7602,7 +7597,7 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
 	hugetlb_vma_assert_locked(vma);
 	if (sz != PMD_SIZE)
 		return 0;
-	if (!ptdesc_pmd_pts_count(virt_to_ptdesc(ptep)))
+	if (!ptdesc_pmd_is_shared(virt_to_ptdesc(ptep)))
 		return 0;
 
 	pud_clear(pud);
-- 
2.43.5
Re: [PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count
Posted by David Hildenbrand 2 weeks, 1 day ago 
On 16.09.25 02:45, Jane Chu wrote:
> commit 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
> introduced ->pt_share_count dedicated to hugetlb PMD share count tracking,
> but omitted fixing copy_hugetlb_page_range(), leaving the function relying on
> page_count() for tracking that no longer works.
> 
> When lazy page table copy for hugetlb is disabled, that is, revert
> commit bcd51a3c679d ("hugetlb: lazy page table copies in fork()")
> fork()'ing with hugetlb PMD sharing quickly lockup -
> 
> [  239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s!
> [  239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0
> [  239.446631] Call Trace:
> [  239.446633]  <TASK>
> [  239.446636]  _raw_spin_lock+0x3f/0x60
> [  239.446639]  copy_hugetlb_page_range+0x258/0xb50
> [  239.446645]  copy_page_range+0x22b/0x2c0
> [  239.446651]  dup_mmap+0x3e2/0x770
> [  239.446654]  dup_mm.constprop.0+0x5e/0x230
> [  239.446657]  copy_process+0xd17/0x1760
> [  239.446660]  kernel_clone+0xc0/0x3e0
> [  239.446661]  __do_sys_clone+0x65/0xa0
> [  239.446664]  do_syscall_64+0x82/0x930
> [  239.446668]  ? count_memcg_events+0xd2/0x190
> [  239.446671]  ? syscall_trace_enter+0x14e/0x1f0
> [  239.446676]  ? syscall_exit_work+0x118/0x150
> [  239.446677]  ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0
> [  239.446681]  ? clear_bhb_loop+0x30/0x80
> [  239.446684]  ? clear_bhb_loop+0x30/0x80
> [  239.446686]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> There are two options to resolve the potential latent issue:
>    1. warn against PMD sharing in copy_hugetlb_page_range(),
>    2. fix it.
> This patch opts for the second option.
> While at it, simplify the comment, the details are not actually relevant
> anymore.
> 
> Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
> Signed-off-by: Jane Chu <jane.chu@oracle.com>
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>   include/linux/mm_types.h |  5 +++++
>   mm/hugetlb.c             | 15 +++++----------
>   2 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 08bc2442db93..a643fae8a349 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -631,6 +631,11 @@ static inline int ptdesc_pmd_pts_count(struct ptdesc *ptdesc)
>   {
>   	return atomic_read(&ptdesc->pt_share_count);
>   }
> +
> +static inline bool ptdesc_pmd_is_shared(struct ptdesc *ptdesc)
> +{
> +	return !!ptdesc_pmd_pts_count(ptdesc);

No need for the !!, the compiler will do that automatically.

LGTM

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb
Re: [PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count
Posted by Andrew Morton 2 weeks, 2 days ago 
On Mon, 15 Sep 2025 18:45:20 -0600 Jane Chu <jane.chu@oracle.com> wrote:

> commit 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
> introduced ->pt_share_count dedicated to hugetlb PMD share count tracking,
> but omitted fixing copy_hugetlb_page_range(), leaving the function relying on
> page_count() for tracking that no longer works.
> 
> When lazy page table copy for hugetlb is disabled, that is, revert
> commit bcd51a3c679d ("hugetlb: lazy page table copies in fork()")
> fork()'ing with hugetlb PMD sharing quickly lockup -
> 
> [  239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s!
> [  239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0
> [  239.446631] Call Trace:
> [  239.446633]  <TASK>
> [  239.446636]  _raw_spin_lock+0x3f/0x60
> [  239.446639]  copy_hugetlb_page_range+0x258/0xb50
> [  239.446645]  copy_page_range+0x22b/0x2c0
> [  239.446651]  dup_mmap+0x3e2/0x770
> [  239.446654]  dup_mm.constprop.0+0x5e/0x230
> [  239.446657]  copy_process+0xd17/0x1760
> [  239.446660]  kernel_clone+0xc0/0x3e0
> [  239.446661]  __do_sys_clone+0x65/0xa0
> [  239.446664]  do_syscall_64+0x82/0x930
> [  239.446668]  ? count_memcg_events+0xd2/0x190
> [  239.446671]  ? syscall_trace_enter+0x14e/0x1f0
> [  239.446676]  ? syscall_exit_work+0x118/0x150
> [  239.446677]  ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0
> [  239.446681]  ? clear_bhb_loop+0x30/0x80
> [  239.446684]  ? clear_bhb_loop+0x30/0x80
> [  239.446686]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> There are two options to resolve the potential latent issue:
>   1. warn against PMD sharing in copy_hugetlb_page_range(),
>   2. fix it.
> This patch opts for the second option.
> While at it, simplify the comment, the details are not actually relevant
> anymore.

Thanks.  I dropped the v2 patch from mm-hotfixes-stable and added this.

> Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
> Signed-off-by: Jane Chu <jane.chu@oracle.com>
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  include/linux/mm_types.h |  5 +++++
>  mm/hugetlb.c             | 15 +++++----------
>  2 files changed, 10 insertions(+), 10 deletions(-)
> 

It's conventional (and useful) to explain (beneath the "---") what
changed since the previous version.

Here's the v2->v3 diff, which appears to be based on David's review
comments:

--- a/include/linux/mm_types.h~mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count
+++ a/include/linux/mm_types.h
@@ -631,6 +631,11 @@ static inline int ptdesc_pmd_pts_count(s
 {
 	return atomic_read(&ptdesc->pt_share_count);
 }
+
+static inline bool ptdesc_pmd_is_shared(struct ptdesc *ptdesc)
+{
+	return !!ptdesc_pmd_pts_count(ptdesc);
+}
 #else
 static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc)
 {
--- a/mm/hugetlb.c~mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count
+++ a/mm/hugetlb.c
@@ -5595,8 +5595,8 @@ int copy_hugetlb_page_range(struct mm_st
 		}
 
 #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
-		/* If the pagetables are shared don't copy or take references. */
-		if (ptdesc_pmd_pts_count(virt_to_ptdesc(dst_pte)) > 0) {
+		/* If the pagetables are shared, there is nothing to do */
+		if (ptdesc_pmd_is_shared(virt_to_ptdesc(dst_pte))) {
 			addr |= last_addr_mask;
 			continue;
 		}
@@ -7597,7 +7597,7 @@ int huge_pmd_unshare(struct mm_struct *m
 	hugetlb_vma_assert_locked(vma);
 	if (sz != PMD_SIZE)
 		return 0;
-	if (!ptdesc_pmd_pts_count(virt_to_ptdesc(ptep)))
+	if (!ptdesc_pmd_is_shared(virt_to_ptdesc(ptep)))
 		return 0;
 
 	pud_clear(pud);
_
Re: [PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count
Posted by jane.chu@oracle.com 2 weeks, 1 day ago 
On 9/15/2025 8:08 PM, Andrew Morton wrote:
>> Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
>> Signed-off-by: Jane Chu<jane.chu@oracle.com>
>> Reviewed-by: Harry Yoo<harry.yoo@oracle.com>
>> ---
>>   include/linux/mm_types.h |  5 +++++
>>   mm/hugetlb.c             | 15 +++++----------
>>   2 files changed, 10 insertions(+), 10 deletions(-)
>>
> It's conventional (and useful) to explain (beneath the "---") what
> changed since the previous version.
> 
> Here's the v2->v3 diff, which appears to be based on David's review
> comments:

Yes, v3 is entirely based on David's comments.  I will resend with 
comments added.

thanks,
-jane

Patchew 2.1-devel-b67e4c2

© 2016 - 2025 Red Hat, Inc.