Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag

From: Jens Axboe <axboe@kernel.dk>
To: Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <djwong@kernel.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	Bernd Schubert <bschubert@ddn.com>,
	io-uring@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
	dsingh@ddn.com
Subject: Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag
Date: Tue, 18 Apr 2023 19:29:50 -0600	[thread overview]
Message-ID: <74244cb8-abf5-b559-5e9f-24d39c801855@kernel.dk> (raw)
In-Reply-To: <ZDuNqQgpHUw+gi9G@infradead.org>

On 4/15/23 11:54?PM, Christoph Hellwig wrote:
> On Fri, Apr 14, 2023 at 08:36:12AM -0700, Darrick J. Wong wrote:
>> IIUC uring wants to avoid the situation where someone sends 300 writes
>> to the same file, all of which end up in background workers, and all of
>> which then contend on exclusive i_rwsem.  Hence it has some hashing
>> scheme that executes io requests serially if they hash to the same value
>> (which iirc is the inode number?) to prevent resource waste.
>>
>> This flag turns off that hashing behavior on the assumption that each of
>> those 300 writes won't serialize on the other 299 writes, hence it's ok
>> to start up 300 workers.
>>
>> (apologies for precoffee garbled response)
> 
> It might be useful if someone (Jens?) could clearly document the
> assumptions for this flag.

I guess it can be summed up as the common case should not be using
exclusive (per file/inode) locking. If file extensions need exclusive
locking that's less of a concern, as I don't think it's unreasonable to
expect that to require stricter locking.

-- 
Jens Axboe