On Fri, Sep 22, 2023 at 09:18:48AM +1000, Dave Chinner wrote: > On Thu, Sep 21, 2023 at 03:26:28PM -0700, Darrick J. Wong wrote: > > On Wed, Sep 20, 2023 at 11:07:37AM +1000, Dave Chinner wrote: > > > On Tue, Sep 19, 2023 at 05:00:58PM -0700, Darrick J. Wong wrote: > > > > On Tue, Sep 19, 2023 at 03:51:24PM +1000, Dave Chinner wrote: > > > > > On Tue, Sep 19, 2023 at 02:43:32AM +0000, Catherine Hoang wrote: > > > > > The XFS clone implementation takes the IOLOCK_EXCL high up, and > > > > > then lower down it iterates one extent doing the sharing operation. > > > > > It holds the ILOCK_EXCL while it is modifying the extent in both the > > > > > source and destination files, then commits the transaction and drops > > > > > the ILOCKs. > > > > > > > > > > OK, so we have fine-grained ILOCK serialisation during the clone for > > > > > access/modification to the extent list. Excellent, I think we can > > > > > make this work. > > > > > > > > > > So: > > > > > > > > > > 1. take IOLOCK_EXCL like we already do on the source and destination > > > > > files. > > > > > > > > > > 2. Once all the pre work is done, set a "clone in progress" flag on > > > > > the in-memory source inode. > > > > > > > > > > 3. atomically demote the source inode IOLOCK_EXCL to IOLOCK_SHARED. > > > > > > > > > > 4. read IO and the clone serialise access to the extent list via the > > > > > ILOCK. We know this works fine, because that's how the extent list > > > > > access serialisation for concurrent read and write direct IO works. > > > > > > > > > > 5. buffered writes take the IOLOCK_EXCL, so they block until the > > > > > clone completes. Same behaviour as right now, all good. > > > > > > > > I think pnfs layouts and DAX writes also take IOLOCK_EXCL, right? So > > > > once reflink breaks the layouts, we're good there too? > > > > > > I think so. > > > > > > <looks to confirm> > > > > > > The pnfs code in xfs_fs_map_blocks() will reject mappings on any > > > inode marked with shared extents, so I think the fact that we > > > set the inode as having shared extents before we finish > > > xfs_reflink_remap_prep() will cause pnfs mappings to kick out before > > > we even take the IOLOCK. > > > > > > But, regardless of that, both new PNFS mappings and DAX writes use > > > IOLOCK_EXCL, and xfs_ilock2_io_mmap() breaks both PNFS and DAX > > > layouts which will force them to finish what they are doing and sync > > > data before the clone operation grabs the IOLOCK_EXCL. They'll block > > > on the clone holding the IOLOCK from that point onwards, so I think > > > we're good here. > > > > > > hmmmmm. > > > > > > <notes that xfs_ilock2_io_mmap() calls filemap_invalidate_lock_two()> > > > > > > Sigh. > > > > > > That will block buffered reads trying to instantiate new pages > > > in the page cache. However, this isn't why the invalidate lock is > > > held - that's being held to lock out lock page faults (i.e. mmap() > > > access) whilst the clone is running. > > > > > > > > > We really only need to lock out mmap writes, and the only way to do > > > that is to prevent write faults from making progress whilst the > > > clone is running. > > > > > > __xfs_filemap_fault() currently takes XFS_MMAPLOCK_SHARED for write > > > faults - I think we need it to look at the "clone in progress" flag > > > for write faults, too, and use XFS_MMAPLOCK_EXCL in that case. > > > > > > That would then allow us to demote the invalidate lock on the source > > > file the same way we do the IOLOCK, allowing buffered reads to > > > populate the page caceh but have write faults block until the clone > > > completes (as they do now, same as writes). > > > > > > Is there anything else I missed? > > > > I think that's it. I'd wondered how much we really care about reflink > > stalling read faults, but yeah, let's fix both. > > Well, it's not so much about mmap as the fact that holding > invalidate lock exclusive prevents adding or removing folios to the > page cache from any path. Hence the change as I originally proposed > would block the buffered read path trying to add pages to the page > cache the same as it will block the read fault path.... Ah, ok. Catherine: Do you have enough information to get started on a proof of concept? --D > Cheers, > > Dave. > > -- > Dave Chinner > david@xxxxxxxxxxxxx
