Re: [PATCH] nfs: flag as supporting FOP_DONTCACHE

[Jens Axboe]

On 12/18/24 10:16 AM, Mike Snitzer wrote:
> On Fri, Dec 13, 2024 at 08:55:14AM -0700, Jens Axboe wrote:
>> Hi,
>>
>> 5 years ago I posted patches adding support for RWF_UNCACHED, as a way
>> to do buffered IO that isn't page cache persistent. The approach back
>> then was to have private pages for IO, and then get rid of them once IO
>> was done. But that then runs into all the issues that O_DIRECT has, in
>> terms of synchronizing with the page cache.
>>
>> So here's a new approach to the same concent, but using the page cache
>> as synchronization. Due to excessive bike shedding on the naming, this
>> is now named RWF_DONTCACHE, and is less special in that it's just page
>> cache IO, except it prunes the ranges once IO is completed.
>>
>> Why do this, you may ask? The tldr is that device speeds are only
>> getting faster, while reclaim is not. Doing normal buffered IO can be
>> very unpredictable, and suck up a lot of resources on the reclaim side.
>> This leads people to use O_DIRECT as a work-around, which has its own
>> set of restrictions in terms of size, offset, and length of IO. It's
>> also inherently synchronous, and now you need async IO as well. While
>> the latter isn't necessarily a big problem as we have good options
>> available there, it also should not be a requirement when all you want
>> to do is read or write some data without caching.
>>
>> Even on desktop type systems, a normal NVMe device can fill the entire
>> page cache in seconds. On the big system I used for testing, there's a
>> lot more RAM, but also a lot more devices. As can be seen in some of the
>> results in the following patches, you can still fill RAM in seconds even
>> when there's 1TB of it. Hence this problem isn't solely a "big
>> hyperscaler system" issue, it's common across the board.
>>
>> Common for both reads and writes with RWF_DONTCACHE is that they use the
>> page cache for IO. Reads work just like a normal buffered read would,
>> with the only exception being that the touched ranges will get pruned
>> after data has been copied. For writes, the ranges will get writeback
>> kicked off before the syscall returns, and then writeback completion
>> will prune the range. Hence writes aren't synchronous, and it's easy to
>> pipeline writes using RWF_DONTCACHE. Folios that aren't instantiated by
>> RWF_DONTCACHE IO are left untouched. This means you that uncached IO
>> will take advantage of the page cache for uptodate data, but not leave
>> anything it instantiated/created in cache.
>>
>> File systems need to support this. This patchset adds support for the
>> generic read path, which covers file systems like ext4. Patches exist to
>> add support for iomap/XFS and btrfs as well, which sit on top of this
>> series. If RWF_DONTCACHE IO is attempted on a file system that doesn't
>> support it, -EOPNOTSUPP is returned. Hence the user can rely on it
>> either working as designed, or flagging and error if that's not the
>> case. The intent here is to give the application a sensible fallback
>> path - eg, it may fall back to O_DIRECT if appropriate, or just live
>> with the fact that uncached IO isn't available and do normal buffered
>> IO.
>>
>> Adding "support" to other file systems should be trivial, most of the
>> time just a one-liner adding FOP_DONTCACHE to the fop_flags in the
>> file_operations struct.
>>
>> Performance results are in patch 8 for reads, and you can find the write
>> side results in the XFS patch adding support for DONTCACHE writes for
>> XFS:
>>
>> ://git.kernel.dk/cgit/linux/commit/?h=buffered-uncached.9&id=edd7b1c910c5251941c6ba179f44b4c81a089019
>>
>> with the tldr being that I see about a 65% improvement in performance
>> for both, with fully predictable IO times. CPU reduction is substantial
>> as well, with no kswapd activity at all for reclaim when using
>> uncached IO.
>>
>> Using it from applications is trivial - just set RWF_DONTCACHE for the
>> read or write, using pwritev2(2) or preadv2(2). For io_uring, same
>> thing, just set RWF_DONTCACHE in sqe->rw_flags for a buffered read/write
>> operation. And that's it.
>>
>> Patches 1..7 are just prep patches, and should have no functional
>> changes at all. Patch 8 adds support for the filemap path for
>> RWF_DONTCACHE reads, and patches 9..11 are just prep patches for
>> supporting the write side of uncached writes. In the below mentioned
>> branch, there are then patches to adopt uncached reads and writes for
>> xfs, btrfs, and ext4. The latter currently relies on bit of a hack for
>> passing whether this is an uncached write or not through
>> ->write_begin(), which can hopefully go away once ext4 adopts iomap for
>> buffered writes. I say this is a hack as it's not the prettiest way to
>> do it, however it is fully solid and will work just fine.
>>
>> Passes full xfstests and fsx overnight runs, no issues observed. That
>> includes the vm running the testing also using RWF_DONTCACHE on the
>> host. I'll post fsstress and fsx patches for RWF_DONTCACHE separately.
>> As far as I'm concerned, no further work needs doing here.
>>
>> And git tree for the patches is here:
>>
>> https://git.kernel.dk/cgit/linux/log/?h=buffered-uncached.9
>>
>>  include/linux/fs.h             | 21 +++++++-
>>  include/linux/page-flags.h     |  5 ++
>>  include/linux/pagemap.h        |  1 +
>>  include/trace/events/mmflags.h |  3 +-
>>  include/uapi/linux/fs.h        |  6 ++-
>>  mm/filemap.c                   | 97 +++++++++++++++++++++++++++++-----
>>  mm/internal.h                  |  2 +
>>  mm/readahead.c                 | 22 ++++++--
>>  mm/swap.c                      |  2 +
>>  mm/truncate.c                  | 54 ++++++++++---------
>>  10 files changed, 166 insertions(+), 47 deletions(-)
>>
>> Since v6
>> - Rename the PG_uncached flag to PG_dropbehind
>> - Shuffle patches around a bit, most notably so the foliop_uncached
>>   patch goes with the ext4 support
>> - Get rid of foliop_uncached hack for btrfs (Christoph)
>> - Get rid of passing in struct address_space to filemap_create_folio()
>> - Inline invalidate_complete_folio2() in folio_unmap_invalidate() rather
>>   than keep it as a separate helper
>> - Rebase on top of current master
>>
>> -- 
>> Jens Axboe
>>
>>
> 
> 
> Hi Jens,
> 
> You may recall I tested NFS to work with UNCACHED (now DONTCACHE).
> I've rebased the required small changes, feel free to append this to
> your series if you like.
> 
> More work is needed to inform knfsd to selectively use DONTCACHE, but
> that will require more effort and coordination amongst the NFS kernel
> team.

Thanks Mike, I'll add it to the part 2 mix.

-- 
Jens Axboe