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

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Message-ID: <Pine.LNX.4.64.0710222101420.23513@blonde.wat.veritas.com>
Date:	Mon, 22 Oct 2007 21:16:17 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
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
Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland

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.

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

Hugh
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