On Nov 15, 2013, at 9:32, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Thu, Nov 14, 2013 at 03:43:21PM -0500, Trond Myklebust wrote: >> >> Hi Christoph, >> >> Why do we need to protect the page cache here? Using O_DIRECT and the page cache without some kind of application-specific synchronization method is technically not supported, since that violates close-to-open cache consistency. > > As far as I understand close to open matter for synchronizations with > access from different clients. Linux applications do expect tight > synchronization locally. Yes, but it seems to me that if you’re adding locking in order to synchronize O_DIRECT with the page cache, then something is very wrong. It is hard to close the races, and you are really mixing 2 models that shouldn’t coexist. It seems a lot more correct in that case to have the kernel convert those O_DIRECT writes into O_SYNC if we really need page cache consistency. That’s the only way we can really _guarantee_ that consistency. >> What we _do_ want to support, though, is parallel reads and writes to the server by applications that know what they are doing. If we were to only protect the i_dio_count, then we could fix the truncate race, while continuing to allow parallelism. Is there any reason why we cannot do this? > > To just fix that race we could do it. Note that even with the patch we > only do the setup and I/O submission under the lock, and wait for it > outside, similar to what the local filesystems do for aio, but not for > synchronous direct I/O. > > Another good way to speed this up is to use locking scheme XFS uses with > a shared exclusive lock: > > buffered write: exclusive > buffered read: shared > direct I/O exclusive if pagecache is present, then > demote to shared for actual I/O, > shared only if no pages are present on > the file I think we should try to close the truncate race first. Then let’s look at whether or not people need better consistency with the page cache. Cheers, Trond-- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
