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SubjectRe: [PATCH v2] mm: anonymous shared memory naming
On 07.11.22 19:47, Pasha Tatashin wrote:
> Since: commit 9a10064f5625 ("mm: add a field to store names for private
> anonymous memory"), name for private anonymous memory, but not shared
> anonymous, can be set. However, naming shared anonymous memory just as

^ is

> useful for tracking purposes.
>
> Extend the functionality to be able to set names for shared anon.
>
> Sample output:
> /* Create shared anonymous segmenet */

s/segmenet/segment/

> anon_shmem = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> MAP_SHARED | MAP_ANONYMOUS, -1, 0);
> /* Name the segment: "MY-NAME" */
> rv = prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME,
> anon_shmem, SIZE, "MY-NAME");
>
> cat /proc/<pid>/maps (and smaps):
> 7fc8e2b4c000-7fc8f2b4c000 rw-s 00000000 00:01 1024 [anon_shmem:MY-NAME]

What would it have looked like before? Just no additional information?

>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---


[...]

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8bbcccbc5565..06b6fb3277ab 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -699,8 +699,10 @@ static inline unsigned long vma_iter_addr(struct vma_iterator *vmi)
> * paths in userfault.
> */
> bool vma_is_shmem(struct vm_area_struct *vma);
> +bool vma_is_anon_shmem(struct vm_area_struct *vma);
> #else
> static inline bool vma_is_shmem(struct vm_area_struct *vma) { return false; }
> +static inline bool vma_is_anon_shmem(struct vm_area_struct *vma) { return false; }
> #endif
>
> int vma_is_stack_for_current(struct vm_area_struct *vma);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 500e536796ca..08d8b973fb60 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -461,21 +461,11 @@ struct vm_area_struct {
> * For areas with an address space and backing store,
> * linkage into the address_space->i_mmap interval tree.
> *
> - * For private anonymous mappings, a pointer to a null terminated string
> - * containing the name given to the vma, or NULL if unnamed.
> */
> -
> - union {
> - struct {
> - struct rb_node rb;
> - unsigned long rb_subtree_last;
> - } shared;
> - /*
> - * Serialized by mmap_sem. Never use directly because it is
> - * valid only when vm_file is NULL. Use anon_vma_name instead.
> - */
> - struct anon_vma_name *anon_name;
> - };
> + struct {
> + struct rb_node rb;
> + unsigned long rb_subtree_last;
> + } shared;
>

So that effectively grows the size of vm_area_struct. Hm. I'd really
prefer to keep this specific to actual anonymous memory, not extending
it to anonymous files.

Do we have any *actual* users where we don't have an alternative? I
doubt that this is really required.

The simplest approach seems to be to use memfd instead of MAP_SHARED |
MAP_ANONYMOUS. __NR_memfd_create can be passed a name and you get what
you propose here effectively already. Or does anything speak against it?

--
Thanks,

David / dhildenb

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