Re: [PATCH] mm: anonymous shared memory naming

["Kirill A. Shutemov" <kirill@shutemov.name>]

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov