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

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Date:	Mon, 22 Oct 2007 17:04:21 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	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
Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland 

In message <Pine.LNX.4.64.0710222101420.23513@...nde.wat.veritas.com>, Hugh Dickins writes:
> 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?
> 
> 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.

Yes, based on vfs.txt I figured unionfs should return
AOP_WRITEPAGE_ACTIVATE.  But, now that unionfs has ->writepages which won't
even call the lower writepage if BDI_CAP_NO_WRITEBACK is on, then perhaps I
no longer need unionfs_writepage to bother checking for
AOP_WRITEPAGE_ACTIVATE, or even return it up?

But, a future file system _could_ return AOP_WRITEPAGE_ACTIVATE w/o setting
BDI_CAP_NO_WRITEBACK, right?  In that case, unionfs will still need to
handle AOP_WRITEPAGE_ACTIVATE in ->writepage, right?

> I think this is where people use the phrase "go figure" ;)
> 
> Hugh

Erez.
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