Re: [PATCHSET v2 0/3] Improve IOCB_NOWAIT O_DIRECT reads

On Tue, Feb 9, 2021 at 10:25 PM Jens Axboe [off-list ref] wrote:

On 2/9/21 12:55 PM, Andrew Morton wrote:

quoted

On Mon,  8 Feb 2021 19:30:05 -0700 Jens Axboe [off-list ref] wrote:

quoted

Hi,

For v1, see:

https://lore.kernel.org/linux-fsdevel/20210208221829.17247-1-axboe@kernel.dk/ (local)

tldr; don't -EAGAIN IOCB_NOWAIT dio reads just because we have page cache
entries for the given range. This causes unnecessary work from the callers
side, when the IO could have been issued totally fine without blocking on
writeback when there is none.

Seems a good idea.  Obviously we'll do more work in the case where some
writeback needs doing, but we'll be doing synchronous writeout in that
case anyway so who cares.

Right, I think that'll be a round two on top of this, so we can make the
write side happier too. That's a bit more involved...

quoted

Please remind me what prevents pages from becoming dirty during or
immediately after the filemap_range_needs_writeback() check?  Perhaps
filemap_range_needs_writeback() could have a comment explaining what it
is that keeps its return value true after it has returned it!

It's inherently racy, just like it is now. There's really no difference
there, and I don't think there's a way to close that. Even if you
modified filemap_write_and_wait_range() to be non-block friendly,
there's nothing stopping anyone from adding dirty page cache right after
that call.

Jens, do you have some numbers before and after your patchset is applied?

And kindly a test "profile" for FIO :-)?

Thanks.

- Sedat -