Re: [PATCH] mm: anonymous shared memory naming

Pasha Tatashin
2022-11-07T15:59:48Z

On Sun, Nov 6, 2022 at 11:52 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Sun, Nov 06, 2022 at 08:45:44AM -0500, Pasha Tatashin wrote:
> > On Sun, Nov 6, 2022 at 8:34 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
> > >
> > > On Sat, Nov 05, 2022 at 02:53:42AM +0000, Pasha Tatashin wrote:
> > > > Since:
> > > > commit 9a10064f5625 ("mm: add a field to store names for private anonymous
> > > > memory")
> > > >
> > > > We can set names for private anonymous memory but not for shared
> > > > anonymous memory. However, naming shared anonymous memory just as
> > > > useful for tracking purposes.
> > > >
> > > > Extend the functionality to be able to set names for shared anon.
> > > >
> > > > / [anon_shmem:<name>] an anonymous shared memory mapping that has
> > > > been named by userspace
> > > >
> > > > Sample output:
> > > > share = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> > > > MAP_SHARED | MAP_ANONYMOUS, -1, 0);
> > > > rv = prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME,
> > > > share, SIZE, "shared anon");
> > > >
> > > > /proc/<pid>/maps (and smaps):
> > > > 7fc8e2b4c000-7fc8f2b4c000 rw-s 00000000 00:01 1024
> > > > /dev/zero (deleted) [anon_shmem:shared anon]
> > > >
> > > > pmap $(pgrep a.out)
> > > > 254: pub/a.out
> > > > 000056093fab2000 4K r---- a.out
> > > > 000056093fab3000 4K r-x-- a.out
> > > > 000056093fab4000 4K r---- a.out
> > > > 000056093fab5000 4K r---- a.out
> > > > 000056093fab6000 4K rw--- a.out
> > > > 000056093fdeb000 132K rw--- [ anon ]
> > > > 00007fc8e2b4c000 262144K rw-s- zero (deleted) [anon_shmem:shared anon]
> > > >
> > > > Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> > > > ---
> > > > Documentation/filesystems/proc.rst | 4 +++-
> > > > fs/proc/task_mmu.c | 7 ++++---
> > > > include/linux/mm.h | 2 ++
> > > > include/linux/mm_types.h | 27 +++++++++++++--------------
> > > > mm/madvise.c | 7 ++-----
> > > > mm/shmem.c | 13 +++++++++++--
> > > > 6 files changed, 35 insertions(+), 25 deletions(-)
> > > >
> > > > diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> > > > index 898c99eae8e4..8f1e68460da5 100644
> > > > --- a/Documentation/filesystems/proc.rst
> > > > +++ b/Documentation/filesystems/proc.rst
> > > > @@ -431,8 +431,10 @@ is not associated with a file:
> > > > [stack] the stack of the main process
> > > > [vdso] the "virtual dynamic shared object",
> > > > the kernel system call handler
> > > > - [anon:<name>] an anonymous mapping that has been
> > > > + [anon:<name>] a private anonymous mapping that has been
> > > > named by userspace
> > > > + path [anon_shmem:<name>] an anonymous shared memory mapping that has
> > > > + been named by userspace
> > >
> > > I expect it to break existing parsers. If the field starts with '/' it is
> > > reasonable to assume the rest of the string to be a path, but it is not
> > > the case now.
> >
> > This is actually exactly why I kept the "path" part. It stays the same
> > as today for anon-shared memory, but prevents pmap to change
> > anon-shared memory from showing it as simply [anon].
> >
> > Here is what we have today in /proc/<pid>/maps (and smaps):
> > 7fc8e2b4c000-7fc8f2b4c000 rw-s 00000000 00:01 1024 /dev/zero (deleted)
> >
> > So, the path points to /dev/zero but appended with (deleted) mark. The
> > pmap shows the same thing, as it is looking for leading '/' to
> > determine that this is a path.
> >
> > With my change the above changes only when user specifically changed
> > the name like this:
> >
> > 7fc8e2b4c000-7fc8f2b4c000 rw-s 00000000 00:01 1024 /dev/zero
> > (deleted) [USER-SPECIFIED-NAME]
> >
> > So, the path stays, the (deleted) mark stays, and a name is added.
>
> Okay, fair enough.

After thinking about this, it makes sense to remove "path" entirely.
The pmap without arguments will show the user named segment as
"[anon]", but with -X argument it will show the full name:

pmap -X
7fa84fcef000 rw-s 00000000 00:01 1024 262144 0 0 0
0 0 0 0 0
0 0 0 0 0 0 [anon_shmem:named
shared anon]
7fa85fcef000 rw-p 00000000 00:00 0 262144 0 0 0
0 0 0 0 0
0 0 0 0 0 0 [anon:named anon]

In my opinion this is better to stay consistent with regular anon
memory, and also to minimize the chance to surprise the existing
scripts.

Pasha

>
> --
> Kiryl Shutsemau / Kirill A. Shutemov