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From: Jens Axboe <axboe@kernel.dk>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, hch@infradead.org
Subject: Re: [PATCHSET v2 0/3] Improve IOCB_NOWAIT O_DIRECT reads
Date: Tue, 9 Feb 2021 13:11:52 -0700	[thread overview]
Message-ID: <32dba5cc-7878-3b7b-45e4-84690a45a998@kernel.dk> (raw)
In-Reply-To: <20210209115542.3e407e306a4f1af29257c8f6@linux-foundation.org>

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


  reply	other threads:[~2021-02-09 21:21 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-09  2:30 Jens Axboe
2021-02-09  2:30 ` [PATCH 1/3] mm: provide filemap_range_needs_writeback() helper Jens Axboe
2021-02-09  7:47   ` Christoph Hellwig
2021-02-09 14:30     ` Jens Axboe
2021-02-09  2:30 ` [PATCH 2/3] mm: use filemap_range_needs_writeback() for O_DIRECT reads Jens Axboe
2021-02-09  7:48   ` Christoph Hellwig
2021-02-09 14:27     ` Jens Axboe
2021-02-09  2:30 ` [PATCH 3/3] iomap: " Jens Axboe
2021-02-09  7:51   ` Christoph Hellwig
2021-02-09 14:29     ` Jens Axboe
2021-02-09 19:55 ` [PATCHSET v2 0/3] Improve IOCB_NOWAIT " Andrew Morton
2021-02-09 20:11   ` Jens Axboe [this message]
2021-02-10  8:07     ` Sedat Dilek
2021-02-10  8:07       ` Sedat Dilek
2021-02-10 14:47       ` Jens Axboe

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