linux-kernel - Re: [PATCH] VM: Fix the gfp_mask in invalidate_complete

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Date:	Fri, 06 Oct 2006 18:16:30 -0400
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Steve Dickson <SteveD@...hat.com>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] VM: Fix the gfp_mask in invalidate_complete_page2

On Fri, 2006-10-06 at 17:49 -0400, Steve Dickson wrote:
> Trond Myklebust wrote:
> > If try_to_release_page() is called with a zero gfp mask, then the
> > filesystem is effectively denied the possibility of sleeping while
> > attempting to release the page. There doesn't appear to be any valid
> > reason why this should be banned, given that we're not calling this from
> > a memory allocation context.
> >  
> > For this reason, change the gfp_mask argument of the call to GFP_KERNEL.
> >     
> > Note: I am less sure of what the callers of invalidate_complete_page()
> > require, and so this patch does not touch that mask.
> > 
> > Signed-off-by: Trond Myklebust <Trond.Myklebust@...app.com>
> > ---
> > diff --git a/mm/truncate.c b/mm/truncate.c
> > index f4edbc1..49c1ffd 100644
> > --- a/mm/truncate.c
> > +++ b/mm/truncate.c
> > @@ -302,7 +302,7 @@ invalidate_complete_page2(struct address
> >  	if (page->mapping != mapping)
> >  		return 0;
> >  
> > -	if (PagePrivate(page) && !try_to_release_page(page, 0))
> > +	if (PagePrivate(page) && !try_to_release_page(page, GFP_KERNEL))
> >  		return 0;
> >  
> >  	write_lock_irq(&mapping->tree_lock);
> Well I was using mapping_gfp_mask(mapping) as the argument to
> try_to_release_page() which also worked... but isn't this
> just plugging one of many holes?

Yeah using mapping_gfp_mask(mapping) sounds like a better option.

...and yes, there are other callers that need to be audited. I'm
particularly curious about the effect of the call in
block_invalidatepage() on XFS, which has a similar test to ours.

The call in fallback_migrate_page() should probably be using
mapping_gfp_mask() too.

> Meaning try_to_release_page is called
> from a number of places with a zero gfp_mask so shouldn't those
> also be fixed as well OR removed the gfp_mask as an argument as the
> comment at the top of try_to_release_page() alludes to?

Nope. In order to make it work correctly with NFS and XFS, all calls to
try_to_release_page() would have to be allowed to be blocking. The
problem is that shrink_page_list() still wants to call
try_to_release_page() from a memory allocation context.

Cheers,
  Trond
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