linux-ext4 - Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

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Date:   Thu, 16 Mar 2017 08:46:27 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, jack@...e.com, hch@...radead.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        sagi@...mberg.me, avi@...lladb.com, axboe@...nel.dk,
        linux-api@...r.kernel.org, Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 3/8] nowait aio: return if direct write will trigger
 writeback

On 03/16/2017 08:08 AM, Matthew Wilcox wrote:
> On Wed, Mar 15, 2017 at 04:51:02PM -0500, Goldwyn Rodrigues wrote:
>> This introduces a new function filemap_range_has_page() which
>> returns true if the file's mapping has a page within the range
>> mentioned.
> 
> I thought you were going to replace this patch with one that starts
> writeback for these pages but does not wait for them?
> 

As mentioned by Jan, Flags to filemap_write_and_wait_range are
unnecessarily complicated. The AIO-DIO API users who eye for performance
usually are careful with page writes/evictions. As a fallback, they can
(and should) go the wait route (without  IOCB_RW_FLAG_NOWAIT).

Finally, my take on this is that we don't want to perform tasks for a
following system call, which may or may not immediately follow the
current one. May not, because an application (DB) will offload the task
from the CPU thread to the I/O thread in case of -EAGAIN. A system call
should be complete in itself (and do the minimum, what is asked).

-- 
Goldwyn