Messages in this thread | | | From | Laurent Stacul <> | Date | Thu, 19 Aug 2021 12:04:17 +0200 | Subject | Re: XFS/mmap reflink file question |
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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
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