linux-kernel - Re: [PATCH+comment] fix tmpfs BUG and AOP_WRITEPAGE

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Date:	Thu, 25 Oct 2007 07:30:08 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	Andrew Morton <akpm@...ux-foundation.org>, ezk@...sunysb.edu,
	ryan@...nie.org, mhalcrow@...ibm.com, cjwatson@...ntu.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, stable@...nel.org
Subject: Re: [PATCH+comment] fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE

On Thu, 25 Oct 2007, Pekka Enberg wrote:
> On 10/25/07, Hugh Dickins <hugh@...itas.com> wrote:
> > --- 2.6.24-rc1/mm/shmem.c       2007-10-24 07:16:04.000000000 +0100
> > +++ linux/mm/shmem.c    2007-10-24 22:31:09.000000000 +0100
> > @@ -915,6 +915,21 @@ static int shmem_writepage(struct page *
> >         struct inode *inode;
> >
> >         BUG_ON(!PageLocked(page));
> > +       /*
> > +        * shmem_backing_dev_info's capabilities prevent regular writeback or
> > +        * sync from ever calling shmem_writepage; but a stacking filesystem
> > +        * may use the ->writepage of its underlying filesystem, in which case
> 
> I find the above bit somewhat misleading as it implies that the
> !wbc->for_reclaim case can be removed after ecryptfs has similar fix
> as unionfs. Can we just say that while BDI_CAP_NO_WRITEBACK does
> prevent some callers from entering ->writepage(), it's just an
> optimization and ->writepage() must deal with !wbc->for_reclaim case
> properly?

Sorry for being obtuse, but I don't see how that's misleading at all.

ecryptfs already has a (dissimilar) fix in 2.6.24-rc1, not using the
writepage route at all.  But it remains the case that some stacking
filesystem may (would you prefer "might" to "may"?  "may" has a nice
double meaning of "might" and "we'll allow it", but this patch does
indeed allow it) use the ->writepage of its underlying filesystem.

With unionfs also fixed, we don't know of an absolute need for this
patch (and so, on that basis, the !wbc->for_reclaim case could indeed
be removed very soon); but as I see it, the unionfs case has shown
that it's time to future-proof this code against whatever stacking
filesystems come along.  Hence I didn't mention the names of such
filesystems in the source comment.

The !page_mapped assumption has been built in there since earliest
2.4, but it took a while for us to get a way to express it in a BUG.

Hugh
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