[PATCH 6.6.y] mm/mremap: correctly uncharge accountable VMAs in error path

[Sasha Levin <sashal@kernel.org>]

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>

[ Upstream commit 65e722a74df992719676ead85a9e93fb2b5a0983 ]

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>
[ sashal: Replaced vrm_uncharge() with a VM_ACCOUNT-guarded vm_unacct_memory(to_account >> PAGE_SHIFT) call in move_vma(). ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/mremap.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index 0739a31b54eed..26d3cb46f6ec1 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -715,6 +715,19 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 		 */
 		move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
 				 true);
+
+		/*
+		 * 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.
+		 */
+		if (vm_flags & VM_ACCOUNT && !(flags & MREMAP_DONTUNMAP))
+			vm_unacct_memory(to_account >> PAGE_SHIFT);
+
 		vma = new_vma;
 		old_len = new_len;
 		old_addr = new_addr;
-- 
2.53.0