Re: XFS/mmap reflink file question

[Laurent Stacul]

On Tue, Aug 17, 2021 at 03:12:58PM -0700, Vito Caputo wrote:
> On Tue, Aug 17, 2021 at 02:19:12PM +0200, Laurent Stacul wrote:
> > Hello,
> >
> > I spent much time digging into the mmap mechanism and I don't have a clear view
> > on mmap'ing a file and a reflink to this file would be mapped twice in memory
> > (this only applies in case the filesystem supports reflink feature like XFS).
> >
> > To describe my tests, I generate a file stored on an XFS partition and create a
> > reflink of it:
> >
> >     % dd if=/dev/zero of=./output.dat bs=1M count=24
> >     % cp --reflink -v output.dat output2.dat
> >     % xfs_bmap -v output.dat
> >     output.dat:
> >      EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET          TOTAL
> >        0: [0..49151]:      3756776..3805927  0 (3756776..3805927) 49152 100000
> >     % xfs_bmap -v output2.dat
> >     output2.dat:
> >      EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET          TOTAL
> >        0: [0..49151]:      3756776..3805927  0 (3756776..3805927) 49152 100000
> >
> > Then I mmap the first file twice using vmtouch tool:
> >
> >     % vmtouch -l output.dat&
> >     [1] 15870
> >     LOCKED 6144 pages (24M)
> >     % vmtouch -l output.dat&
> >     [2] 15872
> >     LOCKED 6144 pages (24M)
> >     % pmap -X 15872 | grep -e 'Pss' -e 'output' | awk '{if(NR>1)printf("%16s %4s %6s %10s %10s %10s\n", $1, $2, $4, $5, $7, $8)}'
> >          Address Perm Device      Inode        Rss        Pss
> >     7fcbb9eb9000 r--s  fc:10    3755268      24576      12288
> >
> > As we can see the Proportional Set Size is as expected the half of the Resident
> > Set Size because the memory is shared by the two processes. Now, I mmap the
> > reflink `output2.dat' of 'output.dat':
> >
> >     % vmtouch -l output2.dat&
> >     [3] 15892
> >     LOCKED 6144 pages (24M)
> >     % pmap -X 15872 | grep -e 'Pss' -e 'output' | awk '{if(NR>1)printf("%16s %4s %6s %10s %10s %10s\n", $1, $2, $4, $5, $7, $8)}'
> >          Address Perm Device      Inode        Rss        Pss
> >     7fcbb9eb9000 r--s  fc:10    3755268      24576      12288
> >
> > The Pss of mmap'ed file by the first process has not decreased (I expected a
> > value of Rss / 3 because I hoped the memory would have been shared by the 3
> > processes). If I look at the process map of the last process, we can interpret
> > a new memory area was allocated and locked.
> >
> >     % pmap -X 15892 | grep -e 'Pss' -e 'output' | awk '{if(NR>1)printf("%16s %4s %6s %10s %10s %10s\n", $1, $2, $4, $5, $7, $8)}'
> >           Address Perm Device      Inode        Rss        Pss
> >      7f5adc53f000 r--s  fc:10    3755269      24576      24576
> >
> > So my questions:
> > - Why can't we benefit from the memory sharing when reflinked files are mmap'ed
> >   ? It would be great because one application would be, in the context of
> >   containers, the possibility to share some read only areas between container
> >   that are built from the layer diff that are reproducible between images. We
> >   can imagine a layer that brings some shared libraries in an image from a
> >   reproducible FS diff so that containers would not load several times a
> >   library.
> > - I can think of many tricky cases with the behavior I was expecting (especially
> >   if a process has write access to the mapped area), but if you know a way, an
> >   option something to achieve what I am trying to do, I would be glad to hear
> >   it.
> > - Conversely, don't hesitate to tell me my expectation is just crazy.
> >
> > Anyway, I am always looking forward to listening to valuable specialist insights.
> > Thanks in advance,
> >
> > stac
> >
> > PS: Please, add me is CC if this message deserves an answer.
>
> This is one of the major features overlayfs brings to the table over
> reflink's current implementation.
>
> With reflink copies you get distinct inodes and the data sharing
> occurs further down in the fs at the extent level, below the struct
> address_space instances.
>
> If memory serves Dave Chinner has given the issue some thought, but I
> haven't noticed/heard anything in terms of progress there.  Maybe
> he'll see this and chime in...
>
> Regards,
> Vito Caputo

Thanks for your answer. If I understand correctly, reflink feature
cannot be used in the scenario I propose because reflinks are
optimization occuring under the VFS. This makes sense to me and I was
not really confident this had a chance to work.

As you suggest, I will turn my effort on overlay.

Regards,
stac