On Sun, Jan 12, 2025 at 06:05:37PM +0800, Chi Zhiling wrote: > On 2025/1/11 01:07, Amir Goldstein wrote: > > On Fri, Jan 10, 2025 at 12:28 AM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > > > > On Wed, Jan 08, 2025 at 09:35:47AM -0800, Darrick J. Wong wrote: > > > > On Wed, Jan 08, 2025 at 03:43:04PM +0800, Chi Zhiling wrote: > > > > > On 2025/1/7 20:13, Amir Goldstein wrote: > > > > > > Dave's answer to this question was that there are some legacy applications > > > > > > (database applications IIRC) on production systems that do rely on the fact > > > > > > that xfs provides this semantics and on the prerequisite that they run on xfs. > > > > > > > > > > > > However, it was noted that: > > > > > > 1. Those application do not require atomicity for any size of IO, they > > > > > > typically work in I/O size that is larger than block size (e.g. 16K or 64K) > > > > > > and they only require no torn writes for this I/O size > > > > > > 2. Large folios and iomap can usually provide this semantics via folio lock, > > > > > > but application has currently no way of knowing if the semantics are > > > > > > provided or not > > > > > > > > > > To be honest, it would be best if the folio lock could provide such > > > > > semantics, as it would not cause any potential problems for the > > > > > application, and we have hope to achieve concurrent writes. > > > > > > > > > > However, I am not sure if this is easy to implement and will not cause > > > > > other problems. > > > > > > > > Assuming we're not abandoning POSIX "Thread Interactions with Regular > > > > File Operations", you can't use the folio lock for coordination, for > > > > several reasons: > > > > > > > > a) Apps can't directly control the size of the folio in the page cache > > > > > > > > b) The folio size can (theoretically) change underneath the program at > > > > any time (reclaim can take your large folio and the next read gets a > > > > smaller folio) > > > > > > > > c) If your write crosses folios, you've just crossed a synchronization > > > > boundary and all bets are off, though all the other filesystems behave > > > > this way and there seem not to be complaints > > > > > > > > d) If you try to "guarantee" folio granularity by messing with min/max > > > > folio size, you run the risk of ENOMEM if the base pages get fragmented > > > > > > > > I think that's why Dave suggested range locks as the correct solution to > > > > this; though it is a pity that so far nobody has come up with a > > > > performant implementation. > > > > > > Yes, that's a fair summary of the situation. > > > > > > That said, I just had a left-field idea for a quasi-range lock > > > that may allow random writes to run concurrently and atomically > > > with reads. > > > > > > Essentially, we add an unsigned long to the inode, and use it as a > > > lock bitmap. That gives up to 64 "lock segments" for the buffered > > > write. We may also need a "segment size" variable.... > > > > > > The existing i_rwsem gets taken shared unless it is an extending > > > write. > > > > > > For a non-extending write, we then do an offset->segment translation > > > and lock that bit in the bit mask. If it's already locked, we wait > > > on the lock bit. i.e. shared IOLOCK, exclusive write bit lock. > > > > > > The segments are evenly sized - say a minimum of 64kB each, but when > > > EOF is extended or truncated (which is done with the i_rwsem held > > > exclusive) the segment size is rescaled. As nothing can hold bit > > > locks while the i_rwsem is held exclusive, this will not race with > > > anything. > > > > > > If we are doing an extending write, we take the i_rwsem shared > > > first, then check if the extension will rescale the locks. If lock > > > rescaling is needed, we have to take the i_rwsem exclusive to do the > > > EOF extension. Otherwise, the bit lock that covers EOF will > > > serialise file extensions so it can be done under a shared i_rwsem > > > safely. > > > > > > This will allow buffered writes to remain atomic w.r.t. each other, > > > and potentially allow buffered reads to wait on writes to the same > > > segment and so potentially provide buffered read vs buffered write > > > atomicity as well. > > > > > > If we need more concurrency than an unsigned long worth of bits for > > > buffered writes, then maybe we can enlarge the bitmap further. > > > > > > I suspect this can be extended to direct IO in a similar way to > > > buffered reads, and that then opens up the possibility of truncate > > > and fallocate() being able to use the bitmap for range exclusion, > > > too. > > > > > > The overhead is likely minimal - setting and clearing bits in a > > > bitmap, as opposed to tracking ranges in a tree structure.... > > > > > > Thoughts? > > > > I think that's a very neat idea, but it will not address the reference > > benchmark. > > The reference benchmark I started the original report with which is similar > > to my understanding to the benchmark that Chi is running simulates the > > workload of a database writing with buffered IO. > > > > That means a very large file and small IO size ~64K. > > Leaving the probability of intersecting writes in the same segment quite high. > > > > Can we do this opportunistically based on available large folios? > > If IO size is within an existing folio, use the folio lock and IOLOCK_SHARED > > if it is not, use IOLOCK_EXCL? > > > > for a benchmark that does all buffered IO 64K aligned, wouldn't large folios > > naturally align to IO size and above? > > > > Great, I think we're getting close to aligning our thoughts. > > IMO, we shouldn't use a shared lock for write operations that are > larger than page size. > > I believe the current issue is that when acquiring the i_rwsem lock, > we have no way of knowing the size of a large folio [1] (as Darrick > mentioned earlier), so we can't determine if only one large folio will > be written. > > There's only one certainty: if the IO size fits within one page size, > it will definitely fit within one large folio. > > So for now, we can only use IOLOCK_SHARED if we verify that the IO fits > within page size. For filesystems that /do/ support large folios (xfs), I suppose you could have it tell iomap that it only took i_rwsem in shared mode; and then the iomap buffered write implementation could proceed if it got a folio covering the entire write range, or return some magic code that means "take i_rwsem in exclusive mode and try again". Though you're correct that we should always take IOLOCK_EXCL if the write size is larger than whatever the max folio size is for that file. --D > [1]: Maybe we can find a way to obtain the size of a folio from the page > cache, but it might come with some performance costs. > > > Thanks, > Chi Zhiling > >
