Re: [PATCH v2 1/4] mm, madvise: simplify anon_name handling

[David Hildenbrand]

On 24.06.25 15:03, Vlastimil Babka wrote:
> Since the introduction in 9a10064f5625 ("mm: add a field to store names
> for private anonymous memory") the code to set anon_name on a vma has
> been using madvise_update_vma() to call replace_anon_vma_name(). Since
> the former is called also by a number of other madvise behaviours that
> do not set a new anon_name, they have been passing the existing
> anon_name of the vma to make replace_vma_anon_name() a no-op.
>
> This is rather wasteful as it needs anon_vma_name_eq() to determine the
> no-op situations, and checks for when replace_vma_anon_name() is allowed
> (the vma is anon/shmem) duplicate the checks already done earlier in
> madvise_vma_behavior(). It has also lead to commit 942341dcc574 ("mm:
> fix use-after-free when anon vma name is used after vma is freed")
> adding anon_name refcount get/put operations exactly to the cases that
> actually do not change anon_name - just so the replace_vma_anon_name()
> can keep safely determining it has nothing to do.
>
> The recent madvise cleanups made this suboptimal handling very obvious,
> but happily also allow for an easy fix. madvise_update_vma() now has the
> complete information whether it's been called to set a new anon_name, so
> stop passing it the existing vma's name and doing the refcount get/put
> in its only caller madvise_vma_behavior().
>
> In madvise_update_vma() itself, limit calling of replace_anon_vma_name()
> only to cases where we are setting a new name, otherwise we know it's a
> no-op. We can rely solely on the __MADV_SET_ANON_VMA_NAME behaviour and
> can remove the duplicate checks for vma being anon/shmem that were done
> already in madvise_vma_behavior().
>
> Additionally, by using vma_modify_flags() when not modifying the
> anon_name, avoid explicitly passing the existing vma's anon_name and
> storing a pointer to it in struct madv_behavior or a local variable.
> This prevents the danger of accessing a freed anon_name after vma
> merging, previously fixed by commit 942341dcc574.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>   mm/madvise.c | 37 +++++++++++++------------------------
>   1 file changed, 13 insertions(+), 24 deletions(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 4491bf080f55d6d1aeffb2ff0b8fdd28904af950..fca0e9b3e844ad766e83ac04cc0d7f4099c74005 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -176,25 +176,29 @@ static int replace_anon_vma_name(struct vm_area_struct *vma,
>   }
>   #endif /* CONFIG_ANON_VMA_NAME */
>   /*
> - * Update the vm_flags on region of a vma, splitting it or merging it as
> - * necessary.  Must be called with mmap_lock held for writing;
> - * Caller should ensure anon_name stability by raising its refcount even when
> - * anon_name belongs to a valid vma because this function might free that vma.
> + * Update the vm_flags and/or anon_name on region of a vma, splitting it or
> + * merging it as necessary. Must be called with mmap_lock held for writing.
>    */
>   static int madvise_update_vma(vm_flags_t new_flags,
>   		struct madvise_behavior *madv_behavior)
>   {
> -	int error;
>   	struct vm_area_struct *vma = madv_behavior->vma;
>   	struct madvise_behavior_range *range = &madv_behavior->range;
>   	struct anon_vma_name *anon_name = madv_behavior->anon_name;
> +	bool set_new_anon_name = madv_behavior->behavior == __MADV_SET_ANON_VMA_NAME;
>   	VMA_ITERATOR(vmi, madv_behavior->mm, range->start);
>   
> -	if (new_flags == vma->vm_flags && anon_vma_name_eq(anon_vma_name(vma), anon_name))
> +	if (new_flags == vma->vm_flags && (!set_new_anon_name ||
> +			anon_vma_name_eq(anon_vma_name(vma), anon_name)))
>   		return 0;
>   
> -	vma = vma_modify_flags_name(&vmi, madv_behavior->prev, vma,
> +	if (set_new_anon_name)
> +		vma = vma_modify_flags_name(&vmi, madv_behavior->prev, vma,
>   			range->start, range->end, new_flags, anon_name);
> +	else
> +		vma = vma_modify_flags(&vmi, madv_behavior->prev, vma,
> +			range->start, range->end, new_flags);
> +
>   	if (IS_ERR(vma))
>   		return PTR_ERR(vma);
>   
> @@ -203,11 +207,8 @@ 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);
>   	vm_flags_reset(vma, new_flags);
> -	if (!vma->vm_file || vma_is_anon_shmem(vma)) {
> -		error = replace_anon_vma_name(vma, anon_name);
> -		if (error)
> -			return error;
> -	}
> +	if (set_new_anon_name)
> +		return replace_anon_vma_name(vma, anon_name);

Took me a second to find where this is already checked (-> 
madvise_vma_behavior()).

:)

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb