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hugetlb: only adjust reservation during unmapping if mapcount is 0

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Since df7a6d1f64, __unmap_hugepage_range can adjust reservations.  In
the case of folio mapped in both a parent and a child, if the parent
unmaps the range first, the reservation adjustment will result in an
underflow of the reserved count.  Once the child unmaps the range, the
count is restored.  Change __unmap_hugepage_range() to check the mapcount
before adjusting the reservation.

Link: https://lore.kernel.org/all/alEJkwn5VlTTH_ZX@bender.morinfr.org/
Link: https://lore.kernel.org/amkC_1Ya6OiUoiLZ@bender.morinfr.org
Fixes: df7a6d1f64 ("mm/hugetlb: restore the reservation if needed")
Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Guillaume Morin
2026-07-28 21:29:03 +02:00
committed by Andrew Morton
parent 62e39381b7
commit 5120b1e048
1 changed files with 6 additions and 2 deletions
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8
mm/hugetlb.c
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@@ -5218,6 +5218,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
bool adjust_reservation;
unsigned long last_addr_mask;
i_mmap_assert_write_locked(vma->vm_file->f_mapping);
WARN_ON(!is_vm_hugetlb_page(vma));
BUG_ON(start & ~huge_page_mask(h));
BUG_ON(end & ~huge_page_mask(h));
@@ -5309,7 +5310,10 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
/*
* Restore the reservation for anonymous page, otherwise the
* backing page could be stolen by someone.
* backing page could be stolen by someone. Restore only on the
* last unmap, otherwise the owner could empty its resv map
* while the folio is still mapped by a child. Note that holding
* i_mmap_lock_write is needed to check the number of mappings.
* If there we are freeing a surplus, do not set the restore
* reservation bit.
*/
@@ -5317,7 +5321,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
spin_lock_irq(&hugetlb_lock);
if (!h->surplus_huge_pages && __vma_private_lock(vma) &&
folio_test_anon(folio)) {
!folio_mapped(folio) && folio_test_anon(folio)) {
folio_set_hugetlb_restore_reserve(folio);
/* Reservation to be adjusted after the spin lock */
adjust_reservation = true;