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

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Date:	Sun, 7 Oct 2007 18:58:54 -0700
From:	"Ryan Finnie" <ryan@...nie.org>
To:	"Pekka J Enberg" <penberg@...helsinki.fi>
Cc:	"Erez Zadok" <ezk@...sunysb.edu>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, "Colin Watson" <cjwatson@...ntu.com>
Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland

On 10/7/07, Pekka J Enberg <penberg@...helsinki.fi> wrote:
> On 10/7/07, Erez Zadok <ezk@...sunysb.edu> wrote:
> > Anyway, some Ubuntu users of Unionfs reported that msync(2) sometimes
> > returns AOP_WRITEPAGE_ACTIVATE (decimal 524288) back to userland.
> > Therefore, some user programs fail, esp. if they're written such as
> > this:
>
...
> It's a kernel bug. AOP_WRITEPAGE_ACTIVATE is a hint to the VM to avoid
> writeback of the page in the near future. I wonder if it's enough that we
> change the return value to zero from
> mm/page-writeback.c:write_cache_pages() in case we hit AOP_WRITEPAGE_ACTIVE...

Doesn't appear to be enough.  I can't figure out why (since it appears
write_cache_pages bubbles up directly to sys_msync), but with that
patch applied, in my test case[1], msync returns -1 EIO.  However,
with the exact same kernel without that patch applied, msync returns
524288 (AOP_WRITEPAGE_ACTIVATE).  But as your patch specifically flips
524288 to 0, I can't figure out how it eventually returns  -1 EIO.

Ryan

[1] "apt-get check" on a unionfs2 mount backed by tmpfs over cdrom,
standard livecd setup
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