| Message ID | 1470181226-20935-1-git-send-email-david@fromorbit.com (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
On Wed, Aug 03, 2016 at 09:40:26AM +1000, Dave Chinner wrote: > From: Dave Chinner <dchinner@redhat.com> > > When we do DAX IO, we try to invalidate the entire page cache held > on the file. This is incorrect as it will trash the entire mapping > tree that now tracks dirty state in exceptional entries in the radix > tree slots. > > What we are trying to do is remove cached pages (e.g from reads > into holes) that sit in the radix tree over the range we are about > to write to. Hence we should just limit the invalidation to the > range we are about to overwrite. Looks fine (for a broad defintion of "fine"): Reviewed-by: Christoph Hellwig <hch@lst.de> > + * XXX: This is racy against mmap, and there's nothing we can do about > + * it. dax_do_io() should really do this invalidation internally as > + * it will know if we've allocated over a holei for this specific IO and > + * if so it needs to update the mapping tree and invalidate existing > + * PTEs over the newly allocated range. Remove this invalidation when > + * dax_do_io() is fixed up. FYI, I've got a basically working version of an iomap based DAX I/O path (still fails a few corner cases), and I'll see if I can add that to it.
On Wed 03-08-16 09:40:26, Dave Chinner wrote: > From: Dave Chinner <dchinner@redhat.com> > > When we do DAX IO, we try to invalidate the entire page cache held > on the file. This is incorrect as it will trash the entire mapping > tree that now tracks dirty state in exceptional entries in the radix > tree slots. > > What we are trying to do is remove cached pages (e.g from reads > into holes) that sit in the radix tree over the range we are about > to write to. Hence we should just limit the invalidation to the > range we are about to overwrite. The patch looks good. Just one comment below. > > Reported-by: Jan Kara <jack@suse.cz> > Signed-off-by: Dave Chinner <dchinner@redhat.com> > --- > fs/xfs/xfs_file.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c > index ed95e5b..e612a02 100644 > --- a/fs/xfs/xfs_file.c > +++ b/fs/xfs/xfs_file.c > @@ -741,9 +741,20 @@ xfs_file_dax_write( > * page is inserted into the pagecache when we have to serve a write > * fault on a hole. It should never be dirtied and can simply be > * dropped from the pagecache once we get real data for the page. > + * > + * XXX: This is racy against mmap, and there's nothing we can do about > + * it. dax_do_io() should really do this invalidation internally as > + * it will know if we've allocated over a holei for this specific IO and > + * if so it needs to update the mapping tree and invalidate existing > + * PTEs over the newly allocated range. Remove this invalidation when > + * dax_do_io() is fixed up. And would it be OK for XFS if dax_do_io() actually invalidated page cache / PTEs under just XFS_IOLOCK_SHARED? Because currently you seem to be careful to call invalidate_inode_pages2() only when holding the lock exclusively and then demote it to a shared one when calling dax_do_io(). Honza > */ > if (mapping->nrpages) { > - ret = invalidate_inode_pages2(mapping); > + loff_t end = iocb->ki_pos + iov_iter_count(from) - 1; > + > + ret = invalidate_inode_pages2_range(mapping, > + iocb->ki_pos >> PAGE_SHIFT, > + end >> PAGE_SHIFT); > WARN_ON_ONCE(ret); > } > > -- > 2.8.0.rc3 >
On Wed, Aug 03, 2016 at 05:34:37PM +0200, Jan Kara wrote: > On Wed 03-08-16 09:40:26, Dave Chinner wrote: > > From: Dave Chinner <dchinner@redhat.com> > > > > When we do DAX IO, we try to invalidate the entire page cache held > > on the file. This is incorrect as it will trash the entire mapping > > tree that now tracks dirty state in exceptional entries in the radix > > tree slots. > > > > What we are trying to do is remove cached pages (e.g from reads > > into holes) that sit in the radix tree over the range we are about > > to write to. Hence we should just limit the invalidation to the > > range we are about to overwrite. > > The patch looks good. Just one comment below. > > > > > Reported-by: Jan Kara <jack@suse.cz> > > Signed-off-by: Dave Chinner <dchinner@redhat.com> > > --- > > fs/xfs/xfs_file.c | 13 ++++++++++++- > > 1 file changed, 12 insertions(+), 1 deletion(-) > > > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c > > index ed95e5b..e612a02 100644 > > --- a/fs/xfs/xfs_file.c > > +++ b/fs/xfs/xfs_file.c > > @@ -741,9 +741,20 @@ xfs_file_dax_write( > > * page is inserted into the pagecache when we have to serve a write > > * fault on a hole. It should never be dirtied and can simply be > > * dropped from the pagecache once we get real data for the page. > > + * > > + * XXX: This is racy against mmap, and there's nothing we can do about > > + * it. dax_do_io() should really do this invalidation internally as > > + * it will know if we've allocated over a holei for this specific IO and > > + * if so it needs to update the mapping tree and invalidate existing > > + * PTEs over the newly allocated range. Remove this invalidation when > > + * dax_do_io() is fixed up. > > And would it be OK for XFS if dax_do_io() actually invalidated page cache / > PTEs under just XFS_IOLOCK_SHARED? Because currently you seem to be careful > to call invalidate_inode_pages2() only when holding the lock exclusively > and then demote it to a shared one when calling dax_do_io(). That really only exists to prevent multiple IOs trying to do invalidation at once. In the direct IO code, we don't want multiple page cache flushers running at once - one is enough - so we serialise on that state knowing that once the invalidation is done the remaining EXCL lock waiters will pass straight through. For DAX, I don't think that's a problem - the invalidation is ranged, and it's unlikely there will be overlaps, and mapping/pte invalidation is done under fine grained locks so we don't have to worry about races there, either. So it seems fine to me to do this under a SHARED lock. It will still serialise against truncate and other extent manipulation operations, and that's mainly what we care about here. Cheers, Dave.
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c index ed95e5b..e612a02 100644 --- a/fs/xfs/xfs_file.c +++ b/fs/xfs/xfs_file.c @@ -741,9 +741,20 @@ xfs_file_dax_write( * page is inserted into the pagecache when we have to serve a write * fault on a hole. It should never be dirtied and can simply be * dropped from the pagecache once we get real data for the page. + * + * XXX: This is racy against mmap, and there's nothing we can do about + * it. dax_do_io() should really do this invalidation internally as + * it will know if we've allocated over a holei for this specific IO and + * if so it needs to update the mapping tree and invalidate existing + * PTEs over the newly allocated range. Remove this invalidation when + * dax_do_io() is fixed up. */ if (mapping->nrpages) { - ret = invalidate_inode_pages2(mapping); + loff_t end = iocb->ki_pos + iov_iter_count(from) - 1; + + ret = invalidate_inode_pages2_range(mapping, + iocb->ki_pos >> PAGE_SHIFT, + end >> PAGE_SHIFT); WARN_ON_ONCE(ret); }