On Fri, Mar 21, 2025 at 07:43:09AM +0530, Ritesh Harjani wrote: > "Darrick J. Wong" <djwong@xxxxxxxxxx> writes: > > > On Fri, Mar 21, 2025 at 12:16:28AM +0530, Ritesh Harjani wrote: > >> Luis Chamberlain <mcgrof@xxxxxxxxxx> writes: > >> > >> > We've been constrained to a max single 512 KiB IO for a while now on x86_64. > >> > This is due to the number of DMA segments and the segment size. With LBS the > >> > segments can be much bigger without using huge pages, and so on a 64 KiB > >> > block size filesystem you can now see 2 MiB IOs when using buffered IO. > >> > But direct IO is still crippled, because allocations are from anonymous > >> > memory, and unless you are using mTHP you won't get large folios. mTHP > >> > is also non-deterministic, and so you end up in a worse situation for > >> > direct IO if you want to rely on large folios, as you may *sometimes* > >> > end up with large folios and sometimes you might not. IO patterns can > >> > therefore be erratic. > >> > > >> > As I just posted in a simple RFC [0], I believe the two step DMA API > >> > helps resolve this. Provided we move the block integrity stuff to the > >> > new DMA API as well, the only patches really needed to support larger > >> > IOs for direct IO for NVMe are: > >> > > >> > iomap: use BLK_MAX_BLOCK_SIZE for the iomap zero page > >> > blkdev: lift BLK_MAX_BLOCK_SIZE to page cache limit > >> > >> Maybe some naive questions, however I would like some help from people > >> who could confirm if my understanding here is correct or not. > >> > >> Given that we now support large folios in buffered I/O directly on raw > >> block devices, applications must carefully serialize direct I/O and > >> buffered I/O operations on these devices, right? > >> > >> IIUC. until now, mixing buffered I/O and direct I/O (for doing I/O on > >> /dev/xxx) on separate boundaries (blocksize == pagesize) worked fine, > >> since direct I/O would only invalidate its corresponding page in the > >> page cache. This assumes that both direct I/O and buffered I/O use the > >> same blocksize and pagesize (e.g. both using 4K or both using 64K). > >> However with large folios now introduced in the buffered I/O path for > >> block devices, direct I/O may end up invalidating an entire large folio, > >> which could span across a region where an ongoing direct I/O operation > > > > I don't understand the question. Should this read ^^^ "buffered"? > > oops, yes. > > > As in, directio submits its write bio, meanwhile another thread > > initiates a buffered write nearby, the write gets a 2MB folio, and > > then the post-write invalidation knocks down the entire large folio? > > Even though the two ranges written are (say) 256k apart? > > > > Yes, Darrick. That is my question. > > i.e. w/o large folios in block devices one could do direct-io & > buffered-io in parallel even just next to each other (assuming 4k pagesize). > > |4k-direct-io | 4k-buffered-io | > > > However with large folios now supported in buffered-io path for block > devices, the application cannot submit such direct-io + buffered-io > pattern in parallel. Since direct-io can end up invalidating the folio > spanning over it's 4k range, on which buffered-io is in progress. > > So now applications need to be careful to not submit any direct-io & > buffered-io in parallel with such above patterns on a raw block device, > correct? That is what I would like to confirm. I think that's correct, and kind of horrifying if true. I wonder if ->invalidate_folio might be a reasonable way to clear the uptodate bits on the relevant parts of a large folio without having to split or remove it? --D > > --D > > > >> is taking place. That means, with large folio support in block devices, > >> application developers must now ensure that direct I/O and buffered I/O > >> operations on block devices are properly serialized, correct? > >> > >> I was looking at posix page [1] and I don't think posix standard defines > >> the semantics for operations on block devices. So it is really upto the > >> individual OS implementation, correct? > >> > >> And IIUC, what Linux recommends is to never mix any kind of direct-io > >> and buffered-io when doing I/O on raw block devices, but I cannot find > >> this recommendation in any Documentation? So can someone please point me > >> one where we recommend this? > > And this ^^^ > > > -ritesh > > >> > >> [1]: https://pubs.opengroup.org/onlinepubs/9799919799/ > >> > >> > >> -ritesh > >> > >> > > >> > The other two nvme-pci patches in that series are to just help with > >> > experimentation now and they can be ignored. > >> > > >> > It does beg a few questions: > >> > > >> > - How are we computing the new max single IO anyway? Are we really > >> > bounded only by what devices support? > >> > - Do we believe this is the step in the right direction? > >> > - Is 2 MiB a sensible max block sector size limit for the next few years? > >> > - What other considerations should we have? > >> > - Do we want something more deterministic for large folios for direct IO? > >> > > >> > [0] https://lkml.kernel.org/r/20250320111328.2841690-1-mcgrof@xxxxxxxxxx > >> > > >> > Luis > >>
