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mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count

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commit 59d9094df3 ("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
bcd51a3c67 ("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.

Link: https://lkml.kernel.org/r/20250916004520.1604530-1-jane.chu@oracle.com
Fixes: 59d9094df3 ("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>
Acked-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Jane Chu 2025-09-15 18:45:20 -06:00 committed by Andrew Morton
parent 7b7387650d
commit 14967a9c7d
2 changed files with 10 additions and 10 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

5
include/linux/mm_types.h
View file

@ -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)
{

15
mm/hugetlb.c
View file

@ -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);