Date:	Fri, 27 Feb 2015 11:36:43 +0100
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm <linux-mm@...ck.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	linux-s390@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 0/4] enhance shmem process and swap accounting

[CC += linux-api@]

Hello Vlastimil,

Since this is a kernel-user-space API change, please CC linux-api@.
The kernel source file Documentation/SubmitChecklist notes that all
Linux kernel patches that change userspace interfaces should be CCed
to linux-api@...r.kernel.org, so that the various parties who are
interested in API changes are informed. For further information, see
https://www.kernel.org/doc/man-pages/linux-api-ml.html

Cheers,

Michael


On Thu, Feb 26, 2015 at 2:51 PM, Vlastimil Babka <vbabka@...e.cz> wrote:
> This series is based on Jerome Marchand's [1] so let me quote the first
> paragraph from there:
>
> There are several shortcomings with the accounting of shared memory
> (sysV shm, shared anonymous mapping, mapping to a tmpfs file). The
> values in /proc/<pid>/status and statm don't allow to distinguish
> between shmem memory and a shared mapping to a regular file, even
> though theirs implication on memory usage are quite different: at
> reclaim, file mapping can be dropped or write back on disk while shmem
> needs a place in swap. As for shmem pages that are swapped-out or in
> swap cache, they aren't accounted at all.
>
> The original motivation for myself is that a customer found (IMHO rightfully)
> confusing that e.g. top output for process swap usage is unreliable with
> respect to swapped out shmem pages, which are not accounted for.
>
> The fundamental difference between private anonymous and shmem pages is that
> the latter has PTE's converted to pte_none, and not swapents. As such, they are
> not accounted to the number of swapents visible e.g. in /proc/pid/status VmSwap
> row. It might be theoretically possible to use swapents when swapping out shmem
> (without extra cost, as one has to change all mappers anyway), and on swap in
> only convert the swapent for the faulting process, leaving swapents in other
> processes until they also fault (so again no extra cost). But I don't know how
> many assumptions this would break, and it would be too disruptive change for a
> relatively small benefit.
>
> Instead, my approach is to document the limitation of VmSwap, and provide means
> to determine the swap usage for shmem areas for those who are interested and
> willing to pay the price, using /proc/pid/smaps. Because outside of ipcs, I
> don't think it's possible to currently to determine the usage at all.  The
> previous patchset [1] did introduce new shmem-specific fields into smaps
> output, and functions to determine the values. I take a simpler approach,
> noting that smaps output already has a "Swap: X kB" line, where currently X ==
> 0 always for shmem areas. I think we can just consider this a bug and provide
> the proper value by consulting the radix tree, as e.g. mincore_page() does. In the
> patch changelog I explain why this is also not perfect (and cannot be without
> swapents), but still arguably much better than showing a 0.
>
> The last two patches are adapted from Jerome's patchset and provide a VmRSS
> breakdown to VmAnon, VmFile and VmShm in /proc/pid/status. Hugh noted that
> this is a welcome addition, and I agree that it might help e.g. debugging
> process memory usage at albeit non-zero, but still rather low cost of extra
> per-mm counter and some page flag checks. I updated these patches to 4.0-rc1,
> made them respect !CONFIG_SHMEM so that tiny systems don't pay the cost, and
> optimized the page flag checking somewhat.
>
> [1] http://lwn.net/Articles/611966/
>
> Jerome Marchand (2):
>   mm, shmem: Add shmem resident memory accounting
>   mm, procfs: Display VmAnon, VmFile and VmShm in /proc/pid/status
>
> Vlastimil Babka (2):
>   mm, documentation: clarify /proc/pid/status VmSwap limitations
>   mm, proc: account for shmem swap in /proc/pid/smaps
>
>  Documentation/filesystems/proc.txt | 15 +++++++++++++--
>  arch/s390/mm/pgtable.c             |  5 +----
>  fs/proc/task_mmu.c                 | 35 +++++++++++++++++++++++++++++++++--
>  include/linux/mm.h                 | 28 ++++++++++++++++++++++++++++
>  include/linux/mm_types.h           |  9 ++++++---
>  kernel/events/uprobes.c            |  2 +-
>  mm/memory.c                        | 30 ++++++++++--------------------
>  mm/oom_kill.c                      |  5 +++--
>  mm/rmap.c                          | 15 ++++-----------
>  9 files changed, 99 insertions(+), 45 deletions(-)
>
> --
> 2.1.4
>
> --
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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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