[PATCH 6.12.y] hugetlb: only adjust reservation during unmapping if mapcount

[Guillaume Morin <guillaume@morinfr.org>]

This is a backport of 5120b1e048d48596ffaec1a8412012a91adba73b which
failed to apply to the 6.12 branch

Since df7a6d1f6405, __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: df7a6d1f6405 ("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>
(cherry picked from commit 5120b1e048d48596ffaec1a8412012a91adba73b)
Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
---
 mm/hugetlb.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e910ed0c43d0..185faeaf5c08 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5523,6 +5523,7 @@ 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));
@@ -5613,7 +5614,10 @@ 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.
 		 */
@@ -5621,7 +5625,7 @@ 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(page_folio(page))) {
+		    !folio_mapped(page_folio(page)) && folio_test_anon(page_folio(page))) {
 			folio_set_hugetlb_restore_reserve(page_folio(page));
 			/* Reservation to be adjusted after the spin lock */
 			adjust_reservation = true;
-- 
2.39.1


-- 
Guillaume Morin <guillaume@morinfr.org>