On Mon, Mar 03, 2025 at 11:08:31AM +0000, Lorenzo Stoakes wrote:
> Consider the case of a 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.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Fixes: 6b73cff239e5 ("mm: change munmap splitting order and move_vma()")
> Cc: stable@xxxxxxxxxxxxxxx
> ---
Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
--
Cheers,
Harry
> mm/mremap.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> 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) {
> --
> 2.48.1
>
>
