Re: [PATCH] fs: invalidate page cache after end_io() in dio completion

[Lukas Czerner <lczerner@redhat.com>]

On Mon, Oct 16, 2017 at 07:23:00PM +0800, Eryu Guan wrote:
> On Wed, Oct 04, 2017 at 12:02:01AM +0800, Eryu Guan wrote:
> > Commit 332391a9935d ("fs: Fix page cache inconsistency when mixing
> > buffered and AIO DIO") moved page cache invalidation from
> > iomap_dio_rw() to iomap_dio_complete() for iomap based direct write
> > path, but before the dio->end_io() call, and it re-introdued the bug
> > fixed by commit c771c14baa33 ("iomap: invalidate page caches should
> > be after iomap_dio_complete() in direct write").
> > 
> > I found this because fstests generic/418 started failing on XFS with
> > v4.14-rc3 kernel, which is the regression test for this specific
> > bug.
> > 
> > So similarly, fix it by moving dio->end_io() (which does the
> > unwritten extent conversion) before page cache invalidation, to make
> > sure next buffer read reads the final real allocations not unwritten
> > extents. I also add some comments about why should end_io() go first
> > in case we get it wrong again in the future.
> > 
> > Note that, there's no such problem in the non-iomap based direct
> > write path, because we didn't remove the page cache invalidation
> > after the ->direct_IO() in generic_file_direct_write() call, but I
> > decided to fix dio_complete() too so we don't leave a landmine
> > there, also be consistent with iomap_dio_complete().
> > 
> > Fixes: 332391a9935d ("fs: Fix page cache inconsistency when mixing buffered and AIO DIO")
> > Signed-off-by: Eryu Guan <eguan@redhat.com>
> 
> Ping on this patch, and cc Jan for broader attention.
> 
> It fixed a regression (re-)introduced in v4.14-rc3, and currently only
> XFS was affected (because of the iomap based DIO adoption). I've tested
> it with the reproducer generic/418, LTP and full rounds of fstests runs
> with different feature and block size enabled.
> 
> Thanks,
> Eryu

Ah, right. I did not notice the problem before. Thanks. The patch looks
good.

Reviewed-by: Lukas Czerner <lczerner@redhat.com>

> 
> > ---
> >  fs/direct-io.c | 20 ++++++++++++--------
> >  fs/iomap.c     | 41 ++++++++++++++++++++++++-----------------
> >  2 files changed, 36 insertions(+), 25 deletions(-)
> > 
> > diff --git a/fs/direct-io.c b/fs/direct-io.c
> > index 62cf812ed0e5..1dba6842c349 100644
> > --- a/fs/direct-io.c
> > +++ b/fs/direct-io.c
> > @@ -259,12 +259,24 @@ static ssize_t dio_complete(struct dio *dio, ssize_t ret, bool is_async)
> >  	if (ret == 0)
> >  		ret = transferred;
> >  
> > +	if (dio->end_io) {
> > +		// XXX: ki_pos??
> > +		err = dio->end_io(dio->iocb, offset, ret, dio->private);
> > +		if (err)
> > +			ret = err;
> > +	}
> > +
> >  	/*
> >  	 * Try again to invalidate clean pages which might have been cached by
> >  	 * non-direct readahead, or faulted in by get_user_pages() if the source
> >  	 * of the write was an mmap'ed region of the file we're writing.  Either
> >  	 * one is a pretty crazy thing to do, so we don't support it 100%.  If
> >  	 * this invalidation fails, tough, the write still worked...
> > +	 *
> > +	 * And this page cache invalidation has to be after dio->end_io(), as
> > +	 * some filesystems convert unwritten extents to real allocations in
> > +	 * end_io() when necessary, otherwise a racing buffer read would cache
> > +	 * zeros from unwritten extents.
> >  	 */
> >  	if (ret > 0 && dio->op == REQ_OP_WRITE &&
> >  	    dio->inode->i_mapping->nrpages) {
> > @@ -274,14 +286,6 @@ static ssize_t dio_complete(struct dio *dio, ssize_t ret, bool is_async)
> >  		WARN_ON_ONCE(err);
> >  	}
> >  
> > -	if (dio->end_io) {
> > -
> > -		// XXX: ki_pos??
> > -		err = dio->end_io(dio->iocb, offset, ret, dio->private);
> > -		if (err)
> > -			ret = err;
> > -	}
> > -
> >  	if (!(dio->flags & DIO_SKIP_DIO_COUNT))
> >  		inode_dio_end(dio->inode);
> >  
> > diff --git a/fs/iomap.c b/fs/iomap.c
> > index be61cf742b5e..d4801f8dd4fd 100644
> > --- a/fs/iomap.c
> > +++ b/fs/iomap.c
> > @@ -714,23 +714,9 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
> >  {
> >  	struct kiocb *iocb = dio->iocb;
> >  	struct inode *inode = file_inode(iocb->ki_filp);
> > +	loff_t offset = iocb->ki_pos;
> >  	ssize_t ret;
> >  
> > -	/*
> > -	 * Try again to invalidate clean pages which might have been cached by
> > -	 * non-direct readahead, or faulted in by get_user_pages() if the source
> > -	 * of the write was an mmap'ed region of the file we're writing.  Either
> > -	 * one is a pretty crazy thing to do, so we don't support it 100%.  If
> > -	 * this invalidation fails, tough, the write still worked...
> > -	 */
> > -	if (!dio->error &&
> > -	    (dio->flags & IOMAP_DIO_WRITE) && inode->i_mapping->nrpages) {
> > -		ret = invalidate_inode_pages2_range(inode->i_mapping,
> > -				iocb->ki_pos >> PAGE_SHIFT,
> > -				(iocb->ki_pos + dio->size - 1) >> PAGE_SHIFT);
> > -		WARN_ON_ONCE(ret);
> > -	}
> > -
> >  	if (dio->end_io) {
> >  		ret = dio->end_io(iocb,
> >  				dio->error ? dio->error : dio->size,
> > @@ -742,12 +728,33 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
> >  	if (likely(!ret)) {
> >  		ret = dio->size;
> >  		/* check for short read */
> > -		if (iocb->ki_pos + ret > dio->i_size &&
> > +		if (offset + ret > dio->i_size &&
> >  		    !(dio->flags & IOMAP_DIO_WRITE))
> > -			ret = dio->i_size - iocb->ki_pos;
> > +			ret = dio->i_size - offset;
> >  		iocb->ki_pos += ret;
> >  	}
> >  
> > +	/*
> > +	 * Try again to invalidate clean pages which might have been cached by
> > +	 * non-direct readahead, or faulted in by get_user_pages() if the source
> > +	 * of the write was an mmap'ed region of the file we're writing.  Either
> > +	 * one is a pretty crazy thing to do, so we don't support it 100%.  If
> > +	 * this invalidation fails, tough, the write still worked...
> > +	 *
> > +	 * And this page cache invalidation has to be after dio->end_io(), as
> > +	 * some filesystems convert unwritten extents to real allocations in
> > +	 * end_io() when necessary, otherwise a racing buffer read would cache
> > +	 * zeros from unwritten extents.
> > +	 */
> > +	if (!dio->error &&
> > +	    (dio->flags & IOMAP_DIO_WRITE) && inode->i_mapping->nrpages) {
> > +		int err;
> > +		err = invalidate_inode_pages2_range(inode->i_mapping,
> > +				offset >> PAGE_SHIFT,
> > +				(offset + dio->size - 1) >> PAGE_SHIFT);
> > +		WARN_ON_ONCE(err);
> > +	}
> > +
> >  	inode_dio_end(file_inode(iocb->ki_filp));
> >  	kfree(dio);
> >  
> > -- 
> > 2.13.6
> >