linux-kernel - Re: [RFC PATCH v2 5/5] mm, shmem: Show location of non-resident shmem pages in smaps

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Message-ID: <20140915172923.GA26275@redhat.com>
Date:	Mon, 15 Sep 2014 19:29:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	linux-mm@...ck.org, Randy Dunlap <rdunlap@...radead.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux390@...ibm.com, Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: Re: [RFC PATCH v2 5/5] mm, shmem: Show location of non-resident
	shmem pages in smaps

On 09/15, Jerome Marchand wrote:
>
> On 09/15/2014 06:21 PM, Oleg Nesterov wrote:
> > Hi Jerome,
> >
> > Not sure I understand this patch correctly, will try to read it later.
> > But a couple of nits/questions anyway,
> >
> > On 09/15, Jerome Marchand wrote:
> >>
> >> +The ShmXXX lines only appears for shmem mapping. They show the amount of memory
> >> +from the mapping that is currently:
> >> + - resident in RAM but not mapped into any process (ShmNotMapped)
> >
> > But how can we know that it is not mapped by another process?
>
> Its mapcount is zero.

Ah, yes, I missed the "!count" check. Thanks!

> > And in fact "not mapped" looks confusing (at least to me).
>
> "Not mapped" as "not present in a page table". It does belong to a
> userspace mapping though. I wonder if there is a less ambiguous terminology.

To me "not present in page tables" looks more understandable, but I won't
insist.

> > IIUC it is actually
> > mapped even by this process, just it never tried to fault these (resident or
> > swapped) pages in. Right?
>
> No these pages are in the page cache. This can happen when the only
> process which have accessed these exits or munmap() the mapping.

Yes, yes, I meant that this process didn't touch these pages and thus
pte_none() == T.

> > And I am not sure why we ignore SHMEM_SWAPCACHE...
>
> Hugh didn't like it as it is a small and transient value.

OK, but perhaps update_shmem_stats() should treat it as SHMEM_SWAP.
Nevermind, I leave this to you and Hugh.

Oleg.

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