linux-kernel - Re: [PATCH v2 06/17] mm: doc comment for scary spot in write_one

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Date:   Wed, 12 Apr 2017 07:38:57 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, tytso@....edu, jack@...e.cz,
        neilb@...e.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH v2 06/17] mm: doc comment for scary spot in write_one_page

On Wed, Apr 12, 2017 at 09:01:34AM -0400, Jeff Layton wrote:
> On Wed, 2017-04-12 at 08:06 -0400, Jeff Layton wrote:
> > Not sure what to do here just yet.
> > 
> > Signed-off-by: Jeff Layton <jlayton@...hat.com>
> > ---
> >  mm/page-writeback.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index de0dbf12e2c1..3ac8399dc984 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -2388,6 +2388,12 @@ int write_one_page(struct page *page)
> >  		ret = mapping->a_ops->writepage(page, &wbc);
> >  		if (ret == 0) {
> >  			wait_on_page_writeback(page);
> > +			/*
> > +			 * FIXME: is this racy? What guarantees that PG_error
> > +			 * will still be set once we get around to checking it?
> > +			 * What if writeback fails, but then a read is issued
> > +			 * before we check this, and that calls ClearPageError?
> > +			 */
> >  			if (PageError(page))
> >  				ret = -EIO;
> >  		}
> 
> Ahh, we are always under the page lock here, and this is generally used
> for writing out directory pages anyway. I'm fine with dropping this
> patch unless someone else sees a problem here.

->writepage drops the page lock.  We're still holding a refcount on this
page, but that's not going to prevent read being called.  But maybe the
filesystem won't call read on a page that's marked as PageError?