linux-kernel - Re: [PATCH 4/5] mm, shmem: Add shmem swap memory accounting

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Message-ID: <alpine.LSU.2.11.1407312204000.3912@eggly.anvils>
Date:	Thu, 31 Jul 2014 22:05:06 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Jerome Marchand <jmarchan@...hat.com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-doc@...r.kernel.org,
	Hugh Dickins <hughd@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux390@...ibm.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH 4/5] mm, shmem: Add shmem swap memory accounting

On Tue, 22 Jul 2014, Jerome Marchand wrote:

> Adds get_mm_shswap() which compute the size of swaped out shmem. It
> does so by pagewalking the mm and using the new shmem_locate() function
> to get the physical location of shmem pages.
> The result is displayed in the new VmShSw line of /proc/<pid>/status.
> Use mm_walk an shmem_locate() to account paged out shmem pages.
> 
> It significantly slows down /proc/<pid>/status acccess speed when
> there is a big shmem mapping. If that is an issue, we can drop this
> patch and only display this counter in the inherently slower
> /proc/<pid>/smaps file (cf. next patch).
> 
> Signed-off-by: Jerome Marchand <jmarchan@...hat.com>

Definite NAK to this one.  As you guessed yourself, it is always a
mistake to add one potentially very slow-to-gather number to a stats
file showing a group of quickly gathered numbers.

Is there anything you could do instead?  I don't know if it's worth
the (little) extra mm_struct storage and maintenance, but you could
add a VmShmSize, which shows that subset of VmSize (total_vm) which
is occupied by shmem mappings.

It's ambiguous what to deduce when VmShm is less than VmShmSize:
the difference might be swapped out, it might be holes in the sparse
object, it might be instantiated in the object but never faulted
into the mapping: in general it will be a mix of all of those.
So, sometimes useful info, but easy to be misled by it.

As I say, I don't know if VmShmSize would be worth adding, given its
deficiencies; and it could be worked out from /proc/<pid>/maps anyway.

Hugh
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