FAILED: patch "[PATCH] mm/mremap: correctly uncharge accountable VMAs in error path" failed to apply to 6.6-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.6-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-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 65e722a74df992719676ead85a9e93fb2b5a0983
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107053909-failed-6.6-65e722a74df9@kernel.org' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Thanks,
Sasha

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

From 65e722a74df992719676ead85a9e93fb2b5a0983 Mon Sep 17 00:00:00 2001
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Date: Mon, 19 Oct 2026 16:57:48 +0100
Subject: [PATCH] mm/mremap: correctly uncharge accountable VMAs in error path

The mremap error path is a fragile and unwieldy beast - it reverses the
page table move then updates vma_remap_struct state to swap VMA, length and
address between old and new VMAs in copy_vma_and_data().

If the mremap() operation didn't specify MREMAP_DONTUNMAP then its caller,
move_vma(), invokes unmap_source_vma(), which unmaps the new VMA rather
than the old one.

For accountable VMAs when MREMAP_DONTUNMAP is not specified,
unmap_source_vma() unsets VMA_ACCOUNT_BIT during this unmap in order that
only the delta of a VMA expansion is charged (performed by
vrm_calc_charge() earlier).

Therefore nothing uncharges the expanded VMA size, and there is a permanent
leak in global Committed_AS.

Resolve the issue by uncharging when MREMAP_DONTUNMAP is not specified.

Link: https://lore.kernel.org/20261019-other-mremap-charge-leak-v1-1-d94477fe6545@kernel.org
Fixes: fdbef6149135 ("mm/mremap: don't account pages in vma_to_resize()")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: <stable@vger.kernel.org>
---
 mm/mremap.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index 24827e13e617f..0d06d7f37f19a 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1339,6 +1339,20 @@ static int copy_vma_and_data(struct vma_remap_struct *vrm,
 		pmc_revert.need_rmap_locks = true;
 		move_page_tables(&pmc_revert);
 
+		/*
+		 * When copy_vma_and_data() returns, the new VMA will be
+		 * unmapped rather than the old one.
+		 *
+		 * When an accountable move is being performed, this unmapping
+		 * will not uncharge any expansion, so do that now.
+		 *
+		 * A move is accountable if the VMA is accountable and
+		 * MREMAP_DONTUNMAP is not specified, so only do so if the
+		 * latter is true.
+		 */
+		if (!(vrm->flags & MREMAP_DONTUNMAP))
+			vrm_uncharge(vrm);
+
 		vrm->vma = new_vma;
 		vrm->old_len = vrm->new_len;
 		vrm->addr = vrm->new_addr;
-- 
2.53.0