On Thu, Jul 09, 2020 at 09:09:26AM -0700, Darrick J. Wong wrote:
> On Thu, Jul 09, 2020 at 12:25:27PM +1000, Dave Chinner wrote:
> > iomap: Only invalidate page cache pages on direct IO writes
> >
> > From: Dave Chinner <dchinner@xxxxxxxxxx>
> >
> > The historic requirement for XFS to invalidate cached pages on
> > direct IO reads has been lost in the twisty pages of history - it was
> > inherited from Irix, which implemented page cache invalidation on
> > read as a method of working around problems synchronising page
> > cache state with uncached IO.
>
> Urk.
>
> > XFS has carried this ever since. In the initial linux ports it was
> > necessary to get mmap and DIO to play "ok" together and not
> > immediately corrupt data. This was the state of play until the linux
> > kernel had infrastructure to track unwritten extents and synchronise
> > page faults with allocations and unwritten extent conversions
> > (->page_mkwrite infrastructure). IOws, the page cache invalidation
> > on DIO read was necessary to prevent trivial data corruptions. This
> > didn't solve all the problems, though.
> >
> > There were peformance problems if we didn't invalidate the entire
> > page cache over the file on read - we couldn't easily determine if
> > the cached pages were over the range of the IO, and invalidation
> > required taking a serialising lock (i_mutex) on the inode. This
> > serialising lock was an issue for XFS, as it was the only exclusive
> > lock in the direct Io read path.
> >
> > Hence if there were any cached pages, we'd just invalidate the
> > entire file in one go so that subsequent IOs didn't need to take the
> > serialising lock. This was a problem that prevented ranged
> > invalidation from being particularly useful for avoiding the
> > remaining coherency issues. This was solved with the conversion of
> > i_mutex to i_rwsem and the conversion of the XFS inode IO lock to
> > use i_rwsem. Hence we could now just do ranged invalidation and the
> > performance problem went away.
> >
> > However, page cache invalidation was still needed to serialise
> > sub-page/sub-block zeroing via direct IO against buffered IO because
> > bufferhead state attached to the cached page could get out of whack
> > when direct IOs were issued. We've removed bufferheads from the
> > XFS code, and we don't carry any extent state on the cached pages
> > anymore, and so this problem has gone away, too.
> >
> > IOWs, it would appear that we don't have any good reason to be
> > invalidating the page cache on DIO reads anymore. Hence remove the
> > invalidation on read because it is unnecessary overhead,
> > not needed to maintain coherency between mmap/buffered access and
> > direct IO anymore, and prevents anyone from using direct IO reads
> > from intentionally invalidating the page cache of a file.
> >
> > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > ---
> > fs/iomap/direct-io.c | 33 +++++++++++++++++----------------
> > 1 file changed, 17 insertions(+), 16 deletions(-)
> >
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index ec7b78e6feca..ef0059eb34b5 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -475,23 +475,24 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> > if (ret)
> > goto out_free_dio;
> >
> > - /*
> > - * Try to invalidate cache pages for the range we're direct
> > - * writing. If this invalidation fails, tough, the write will
> > - * still work, but racing two incompatible write paths is a
> > - * pretty crazy thing to do, so we don't support it 100%.
>
> I always wondered about the repeated use of 'write' in this comment
> despite the lack of any sort of WRITE check logic. Seems fine to me,
> let's throw it on the fstests pile and see what happens.
>
> Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> --D
>
> > - */
> > - ret = invalidate_inode_pages2_range(mapping,
> > - pos >> PAGE_SHIFT, end >> PAGE_SHIFT);
> > - if (ret)
> > - dio_warn_stale_pagecache(iocb->ki_filp);
> > - ret = 0;
> > + if (iov_iter_rw(iter) == WRITE) {
> > + /*
> > + * Try to invalidate cache pages for the range we're direct
> > + * writing. If this invalidation fails, tough, the write will
> > + * still work, but racing two incompatible write paths is a
> > + * pretty crazy thing to do, so we don't support it 100%.
> > + */
> > + ret = invalidate_inode_pages2_range(mapping,
> > + pos >> PAGE_SHIFT, end >> PAGE_SHIFT);
> > + if (ret)
> > + dio_warn_stale_pagecache(iocb->ki_filp);
> > + ret = 0;
> >
> > - if (iov_iter_rw(iter) == WRITE && !wait_for_completion &&
> > - !inode->i_sb->s_dio_done_wq) {
> > - ret = sb_init_dio_done_wq(inode->i_sb);
> > - if (ret < 0)
> > - goto out_free_dio;
> > + if (!wait_for_completion &&
> > + !inode->i_sb->s_dio_done_wq) {
> > + ret = sb_init_dio_done_wq(inode->i_sb);
> > + if (ret < 0)
> > + goto out_free_dio;
> > }
> >
> > inode_dio_begin(inode);
