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

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Date:	Sun, 7 Oct 2007 22:58:11 +0300 (EEST)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Erez Zadok <ezk@...sunysb.edu>
cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Ryan Finnie <ryan@...nie.org>,
	Colin Watson <cjwatson@...ntu.com>
Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland

Hi Erez,

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:

[snip]

On 10/7/07, Erez Zadok <ezk@...sunysb.edu> wrote:
> Is this a bug indeed, or are user programs supposed to handleÂ 
> AOP_WRITEPAGE_ACTIVATE (I hope not the latter). If it's a kernel bug, 
> what should the kernel return: a zero, or an -errno (and which one)?

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...

				Pekka 

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 63512a9..717f341 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -672,8 +672,10 @@ retry:
 
 			ret = (*writepage)(page, wbc, data);
 
-			if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE))
+			if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE)) {
 				unlock_page(page);
+				ret = 0;
+			}
 			if (ret || (--(wbc->nr_to_write) <= 0))
 				done = 1;
 			if (wbc->nonblocking && bdi_write_congested(bdi)) {