linux-kernel - Re: Partialy mapped page stays in page cache after unmap

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Message-ID: <CAA_GA1fozH3wA+2YWrCEUN2S=3rSpJ3f829yy8TZFfuh8q-WYQ@mail.gmail.com>
Date:	Wed, 31 Oct 2012 20:52:21 +0800
From:	Bob Liu <lliubbo@...il.com>
To:	chrubis@...e.cz
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hugh Dickins <hughd@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Partialy mapped page stays in page cache after unmap

On Wed, Oct 31, 2012 at 2:24 AM,  <chrubis@...e.cz> wrote:
> Hi!
> I'm currently revisiting mmap related tests in LTP (Linux Test Project)
> and I've came to the tests testing that writes to the partially
> mapped page (at the end of mapping) are carried out correctly.
>
> These tests fails because even after the object is unmapped and the
> file-descriptor closed the pages still stays in the page cache so if
> (possibly another process) opens and maps the file again the whole
> content of the partial page is preserved.
>
> Strictly speaking this is not a bug at least when sticking to regular
> files as POSIX which says that the change is not written out. In this
> case the file content is correct and forcing the data to be written out
> by msync() makes the test pass. The SHM mappings seems to preserve the
> content even after calling msync() which is, in my opinion, POSIX
> violation although a minor one.
>

fsync implemented in SHM is noop_fsync.
May be we should extend it if needed.

> Looking at the test results I have, the file based mmap test worked fine
> on 2.6.5 (or perhaps the page cache was working/setup differently and
> the test succeeded by accidend).
>
> Attached is a stripped down LTP test for the problem, uncommenting the
> msync() makes the test succeed.
>
> I would like to hear your opinions on this problems.
>
> --
> Cyril Hrubis
> chrubis@...e.cz

-- 
Regards,
--Bob
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