linux-kernel - Re: [PATCH] shmem: call set_page

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Date:	Wed, 15 Jul 2009 19:10:12 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] shmem: call set_page_dirty() with locked page

On Wed, Jul 15, 2009 at 02:24:54AM +0800, Hugh Dickins wrote:
> On Tue, 14 Jul 2009, Wu Fengguang wrote:
> > Here set_page_dirty() can be moved into the page lock.
> 
> Indeed it can, but you've forgotten to mention why you think
> that would be a good thing?  All I can see is that it would

Sorry for missing out the rational.  My problem is: the hwpoison code
must make sure whether one page can be dropped without losing data.

> very very slightly increase the page's lock hold time, which
> wouldn't be an improvement: what improvement are you making?

Yes there were nothing wrong. Just to make it align with the general
practice(not rule): pages are normally dirtied inside the page lock.

The noticeable exceptions are mapped pages and pages with buffer_heads
- they could go dirty at any time. Fortunately they will have to be
unmapped/released anyway.

shmem may not be the only remaining exception. But let's fix it first.
I'd be appreciated if someone could name some more exceptions, or some
better criterion on "the data in this page can be recovered".

Thanks,
Fengguang

> > CC: Hugh Dickins <hugh@...itas.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  mm/shmem.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- linux.orig/mm/shmem.c
> > +++ linux/mm/shmem.c
> > @@ -1630,8 +1630,8 @@ shmem_write_end(struct file *file, struc
> >  	if (pos + copied > inode->i_size)
> >  		i_size_write(inode, pos + copied);
> >  
> > -	unlock_page(page);
> >  	set_page_dirty(page);
> > +	unlock_page(page);
> >  	page_cache_release(page);
> >  
> >  	return copied;
--
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