Re: [PATCH 3/4] mm/madvise: preserve VMA_SOFTDIRTY_BIT when changing anonymous names

[Pedro Falcato <pfalcato@suse.de>]

On Fri, Oct 23, 2026 at 02:47:29PM +0100, Lorenzo Stoakes (ARM) wrote:
> From: "David Hildenbrand (Arm)" <david@kernel.org>
> 
> When changing the anonymous VMA name, madvise_update_vma() calls
> vma_modify_name() to possibly merge with adjacent VMAs. We don't call
> vma_modify_flags() and consequently don't obtain the new VMA flags.
> 
> Later in the function we update the VMA flags to the stale new_vma_flags
> we obtained before. This can result in us losing the VMA_SOFTDIRTY_BIT
> flag set on a VMA we just merged with.
> 
> There is no need to update the VMA flags at all, so just exit early. This
> skips calling khugepaged_enter_vma(), but since the flags didn't change
> this is fine.
> 
> Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> [ ljs - Minor edit of commit message, rebased ]
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Pedro Falcato <pfalcato@suse.de>

> ---
>  mm/madvise.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index b2e00efcaa54..e9fa4f71efea 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -177,6 +177,9 @@ static int madvise_update_vma(vm_flags_t new_flags,
>  
>  	/* vm_flags is protected by the mmap_lock held in write mode. */
>  	vma_start_write(vma);
> +	if (set_new_anon_name)
> +		return replace_anon_vma_name(vma, anon_name);

This looks non-obvious and brittle, maybe have a comment here?
Or some more clever solution.

-- 
Pedro