On Wed, Feb 28, 2024 at 09:13:05AM +1100, Dave Chinner wrote: > On Tue, Feb 27, 2024 at 05:07:30AM -0500, Kent Overstreet wrote: > > AFAIK every filesystem allows concurrent direct writes, not just xfs, > > it's _buffered_ writes that we care about here. > > We could do concurrent buffered writes in XFS - we would just use > the same locking strategy as direct IO and fall back on folio locks > for copy-in exclusion like ext4 does. ext4 code doesn't do that. it takes the inode lock in exclusive mode, just like everyone else. > The real question is how much of userspace will that break, because > of implicit assumptions that the kernel has always serialised > buffered writes? What would break? > > If we do a short write because of a page fault (despite previously > > faulting in the userspace buffer), there is no way to completely prevent > > torn writes an atomicity breakage; we could at least try a trylock on > > the inode lock, I didn't do that here. > > As soon as we go for concurrent writes, we give up on any concept of > atomicity of buffered writes (esp. w.r.t reads), so this really > doesn't matter at all. We've already given up buffered write vs. read atomicity, have for a long time - buffered read path takes no locks.
