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

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On 2/9/21 12:55 PM, Andrew Morton wrote:
> On Mon,  8 Feb 2021 19:30:05 -0700 Jens Axboe <axboe@xxxxxxxxx> wrote:
> 
>> Hi,
>>
>> For v1, see:
>>
>> https://lore.kernel.org/linux-fsdevel/20210208221829.17247-1-axboe@xxxxxxxxx/
>>
>> 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...

> 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 Axboe





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