Re: [LSF/MM/BPF TOPIC] breaking the 512 KiB IO boundary on x86_64

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Mar 21, 2025 at 12:16:28AM +0530, Ritesh Harjani wrote:
> Luis Chamberlain <mcgrof@kernel.org> 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"?
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?

--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?
> 
> [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@kernel.org
> >
> >   Luis
>