Re: [PATCH] iomap: Ensure iop->uptodate matches PageUptodate

From: Matthew Wilcox <willy@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Brian Foster <bfoster@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iomap: Ensure iop->uptodate matches PageUptodate
Date: Mon, 27 Jul 2020 00:20:22 +0100	[thread overview]
Message-ID: <20200726232022.GH23808@casper.infradead.org> (raw)
In-Reply-To: <20200726230657.GT2005@dread.disaster.area>

On Mon, Jul 27, 2020 at 09:06:57AM +1000, Dave Chinner wrote:
> On Sun, Jul 26, 2020 at 10:10:52AM +0100, Matthew Wilcox (Oracle) wrote:
> > If the filesystem has block size < page size and we end up calling
> > iomap_page_create() in iomap_page_mkwrite_actor(), the uptodate bits
> > would be zero, which causes us to skip writeback of blocks which are
> > !uptodate in iomap_writepage_map().  This can lead to user data loss.
> 
> I'm still unclear on what condition gets us to
> iomap_page_mkwrite_actor() without already having initialised the
> page correctly. i.e. via a read() or write() call, or the read fault
> prior to ->page_mkwrite() which would have marked the page uptodate
> - that operation should have called iomap_page_create() and
> iomap_set_range_uptodate() on the page....
> 
> i.e. you've described the symptom, but not the cause of the issue
> you are addressing.

I don't know exactly what condition gets us there either.  It must be
possible, or there wouldn't be a call to iomap_page_create() but rather
one to to_iomap_page() like the one in iomap_finish_page_writeback().

> > Found using generic/127 with the THP patches.  I don't think this can be
> > reproduced on mainline using that test (the THP code causes iomap_pages
> > to be discarded more frequently), but inspection shows it can happen
> > with an appropriate series of operations.
> 
> That sequence of operations would be? 
> 
> > Fixes: 9dc55f1389f9 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > ---
> >  fs/iomap/buffered-io.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index a2b3b5455219..f0c5027bf33f 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -53,7 +53,10 @@ iomap_page_create(struct inode *inode, struct page *page)
> >  	atomic_set(&iop->read_count, 0);
> >  	atomic_set(&iop->write_count, 0);
> >  	spin_lock_init(&iop->uptodate_lock);
> > -	bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
> > +	if (PageUptodate(page))
> > +		bitmap_fill(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
> > +	else
> > +		bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
> 
> I suspect this bitmap_fill call belongs in the iomap_page_mkwrite()
> code as is the only code that can call iomap_page_create() with an
> uptodate page. Then iomap_page_create() could just use kzalloc() and
> drop the atomic_set() and bitmap_zero() calls altogether,

Way ahead of you
http://git.infradead.org/users/willy/pagecache.git/commitdiff/5a1de6fc4f815797caa4a2f37c208c67afd7c20b