linux-kernel - Re: msync(2) bug(?), returns AOP_WRITEPAGE

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Date:	Mon, 22 Oct 2007 23:48:37 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Hugh Dickins" <hugh@...itas.com>
Cc:	"Erez Zadok" <ezk@...sunysb.edu>, "Ryan Finnie" <ryan@...nie.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	cjwatson@...ntu.com, linux-mm@...ck.org, neilb@...e.de
Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland

Hi Hugh,

On Mon, 15 Oct 2007, Pekka Enberg wrote:
> > I wonder whether _not setting_ BDI_CAP_NO_WRITEBACK implies that
> > ->writepage() will never return AOP_WRITEPAGE_ACTIVATE for
> > !wbc->for_reclaim case which would explain why we haven't hit this bug
> > before. Hugh, Andrew?

On 10/22/07, Hugh Dickins <hugh@...itas.com> wrote:
> Only ramdisk and shmem have been returning AOP_WRITEPAGE_ACTIVATE.
> Both of those set BDI_CAP_NO_WRITEBACK.  ramdisk never returned it
> if !wbc->for_reclaim.  I contend that shmem shouldn't either: it's
> a special code to get the LRU rotation right, not useful elsewhere.
> Though Documentation/filesystems/vfs.txt does imply wider use.
>
> I think this is where people use the phrase "go figure" ;)

Heh. As far as I can tell, the implication of "wider use" was added by
Neil in commit "341546f5ad6fce584531f744853a5807a140f2a9 Update some
VFS documentation", so perhaps he might know? Neil?

                               Pekka
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