From 397432cab17bccb600fd6c16ed593f1149042268 Mon Sep 17 00:00:00 2001 From: "Lorenzo Stoakes (ARM)" Date: Fri, 28 Aug 2026 12:20:37 +0100 Subject: [PATCH] mm/mremap: account mm->locked_vm correctly for MREMAP_DONTUNMAP When a VMA is mremap()'d with MREMAP_DONTUNMAP set, that results in the VMA being copied, but the source VMA not being unmapped. If the VMA is mlock()'d this is a legal operation, though the source VMA has its VMA_LOCKED_BIT cleared. However this is done in dontunmap_complete(), after mm->locked_vm was incremented via vrm_stat_account(), resulting in double-counting. Worse, this is not even corrected when source VMA is unmapped, due to the VMA_LOCKED_BIT flag having been cleared. This all works fine in the usual mremap() case (without MREMAP_DONTUNMAP), as the source VMA is unmapped with VMA_LOCKED_BIT intact, at which time mm->locked_vm is decremented accordingly. Resolve the issue by invoking vrm_stat_account() only after dontunmap_complete() has run. Note that MREMAP_DONTUNMAP requires old_len == new_len, so no need to account for a delta in size in this case. The bug was introduced by commit b714ccb02a76 ("mm/mremap: complete refactor of move_vma()") which incorrectly reordered the accounting and the clearing of the VMA_LOCKED_BIT flag. Link: https://lore.kernel.org/20260828-mremap-fix-locked-vm-v1-1-c80be7505d1e@kernel.org Fixes: b714ccb02a76 ("mm/mremap: complete refactor of move_vma()") Signed-off-by: Lorenzo Stoakes (ARM) Reported-by: sashiko-bot Closes: https://sashiko.dev/#/patchset/20260825-fix-mremap-dontunmap-pgoff-v1-1-39a40b2c98b3@kernel.org Reported-by: Kunwu Chan Closes: https://lore.kernel.org/all/20260828094823.594279-1-kunwu.chan@linux.dev/ Acked-by: Vlastimil Babka (SUSE) Tested-by: Kunwu Chan Reviewed-by: Kunwu Chan Cc: Jann Horn Cc: Liam R. Howlett Cc: Pedro Falcato Cc: Signed-off-by: Andrew Morton --- mm/mremap.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-) diff --git a/mm/mremap.c b/mm/mremap.c index 2b4b523a86b8..7c368440fafe 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -1355,12 +1355,11 @@ static void dontunmap_complete(struct vma_remap_struct *vrm, if (vma_is_anonymous(vma) && !vma->vm_file) vma_set_pgoff(vma, pgoff_unfaulted); } - - /* Because we won't unmap we don't need to touch locked_vm. */ } static unsigned long move_vma(struct vma_remap_struct *vrm) { + const bool is_dontunmap = vrm->flags & MREMAP_DONTUNMAP; struct mm_struct *mm = current->mm; struct vm_area_struct *new_vma; unsigned long hiwater_vm; @@ -1401,10 +1400,10 @@ static unsigned long move_vma(struct vma_remap_struct *vrm) */ hiwater_vm = mm->hiwater_vm; - vrm_stat_account(vrm, vrm->new_len); - if (unlikely(!err && (vrm->flags & MREMAP_DONTUNMAP))) + if (unlikely(is_dontunmap && !err)) dontunmap_complete(vrm, new_vma); - else + vrm_stat_account(vrm, vrm->new_len); + if (!is_dontunmap || err) unmap_source_vma(vrm); mm->hiwater_vm = hiwater_vm;