[v3,1/7] mm/mremap: correctly handle partial mremap() of VMA starting at 0

Commit Message

Lorenzo Stoakes March 10, 2025, 8:50 p.m. UTC

Consider the case of a partial mremap() (that results in a VMA split) of
an accountable VMA (i.e.  which has the VM_ACCOUNT flag set) whose start
address is zero, with the MREMAP_MAYMOVE flag specified and a scenario
where a move does in fact occur:

       addr  end
        |     |
        v     v
    |-------------|
    |     vma     |
    |-------------|
    0

This move is affected by unmapping the range [addr, end).  In order to
prevent an incorrect decrement of accounted memory which has already been
determined, the mremap() code in move_vma() clears VM_ACCOUNT from the VMA
prior to doing so, before reestablishing it in each of the VMAs
post-split:

    addr  end
     |     |
     v     v
 |---|     |---|
 | A |     | B |
 |---|     |---|

Commit 6b73cff239e5 ("mm: change munmap splitting order and move_vma()")
changed this logic such as to determine whether there is a need to do so
by establishing account_start and account_end and, in the instance where
such an operation is required, assigning them to vma->vm_start and
vma->vm_end.

Later the code checks if the operation is required for 'A' referenced
above thusly:

	if (account_start) {
		...
	}

However, if the VMA described above has vma->vm_start == 0, which is now
assigned to account_start, this branch will not be executed.

As a result, the VMA 'A' above will remain stripped of its VM_ACCOUNT
flag, incorrectly.

The fix is to simply convert these variables to booleans and set them as
required.

Fixes: 6b73cff239e5 ("mm: change munmap splitting order and move_vma()")
Signed-off-by: Lorenzo Stoakes <[e-mail]>
Reviewed-by: Harry Yoo <[e-mail]>
Reviewed-by: Liam R. Howlett <[e-mail]>
Reviewed-by: Vlastimil Babka <[e-mail]>
Cc: <[e-mail]>
---
 mm/mremap.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

Patch

diff --git a/mm/mremap.c b/mm/mremap.c
index cff7f552f909..c3e4c86d0b8d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -705,8 +705,8 @@  static unsigned long move_vma(struct vm_area_struct *vma,
 	unsigned long vm_flags = vma->vm_flags;
 	unsigned long new_pgoff;
 	unsigned long moved_len;
-	unsigned long account_start = 0;
-	unsigned long account_end = 0;
+	bool account_start = false;
+	bool account_end = false;
 	unsigned long hiwater_vm;
 	int err = 0;
 	bool need_rmap_locks;
@@ -790,9 +790,9 @@  static unsigned long move_vma(struct vm_area_struct *vma,
 	if (vm_flags & VM_ACCOUNT && !(flags & MREMAP_DONTUNMAP)) {
 		vm_flags_clear(vma, VM_ACCOUNT);
 		if (vma->vm_start < old_addr)
-			account_start = vma->vm_start;
+			account_start = true;
 		if (vma->vm_end > old_addr + old_len)
-			account_end = vma->vm_end;
+			account_end = true;
 	}
 
 	/*
@@ -832,7 +832,7 @@  static unsigned long move_vma(struct vm_area_struct *vma,
 		/* OOM: unable to split vma, just get accounts right */
 		if (vm_flags & VM_ACCOUNT && !(flags & MREMAP_DONTUNMAP))
 			vm_acct_memory(old_len >> PAGE_SHIFT);
-		account_start = account_end = 0;
+		account_start = account_end = false;
 	}
 
 	if (vm_flags & VM_LOCKED) {