FAILED: patch "[PATCH] hugetlb: only adjust reservation during unmapping if mapcount" failed to apply to 6.12-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 5120b1e048d48596ffaec1a8412012a91adba73b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026090356-destiny-subject-4d56@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5120b1e048d48596ffaec1a8412012a91adba73b Mon Sep 17 00:00:00 2001
From: Guillaume Morin <guillaume@morinfr.org>
Date: Tue, 28 Jul 2026 21:29:03 +0200
Subject: [PATCH] hugetlb: only adjust reservation during unmapping if mapcount
 is 0

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>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 49bf325325c0..7d14511c20ad 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -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;