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Date:   Wed, 8 Jun 2022 08:16:18 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Christoph Hellwig <hch@lst.de>, linux-fsdevel@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH 2/9] btrfs_direct_write(): cleaner way to handle
 generic_write_sync() suppression
Message-ID: <20220608061618.GA7532@lst.de>
References: <Yp7PTZ2nckKDTkKu@zeniv-ca.linux.org.uk> <Yp7PlaCTJF19m2sG@zeniv-ca.linux.org.uk> <20220607044217.GB7887@lst.de> <Yp93nbSBggvxjVqa@zeniv-ca.linux.org.uk>
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On Tue, Jun 07, 2022 at 04:06:53PM +0000, Al Viro wrote:
> > Also the FUA check below needs to check IOMAP_DIO_NOSYNC as
> > well.
> 
> Does it?  AFAICS, we don't really care about REQ_FUA on any requests - what
> btrfs hack tries to avoid is stepping into
>         if (ret > 0 && (dio->flags & IOMAP_DIO_NEED_SYNC))
> 		ret = generic_write_sync(iocb, ret);
> with generic_write_sync() called by btrfs_do_write_iter() after it has
> dropped the lock held through btrfs_direct_write().  Do we want to
> suppress REQ_FUA on the requests generated by __iomap_dio_rw() in
> that case (DSYNC, !SYNC)?  Confused...

Yes.  FUA is an used an an optimization so that we can avid the
generic_write_sync or similar call if a O_DSYNC write doesn't need
any metadata update.  If the caller already does the generic_write_sync
equivalent we don't also need FUA.  It is not actually harmful in that
it gives worse results, but it will kill performance.