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Date:   Tue, 7 Jun 2022 15:34:05 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jan Kara <jack@suse.cz>
Cc:     linux-fsdevel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH 4/9] iocb: delay evaluation of IS_SYNC(...) until we want
 to check IOCB_DSYNC
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On Tue, Jun 07, 2022 at 12:34:50PM +0200, Jan Kara wrote:
> On Tue 07-06-22 04:11:06, Al Viro wrote:
> > New helper to be used instead of direct checks for IOCB_DSYNC:
> > iocb_is_dsync(iocb).  Checks converted, which allows to avoid
> > the IS_SYNC(iocb->ki_filp->f_mapping->host) part (4 cache lines)
> > from iocb_flags() - it's checked in iocb_is_dsync() instead
> > 
> > Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> 
> Does it really matter that much when we have io_is_direct() just two lines
> above which does: IS_DAX(filp->f_mapping->host)?
> 
> Also presumably even if we got rid of the IS_DAX check, we'll need to do
> file->f_mapping->host traversal sooner rather than later anyway so it is
> not clear to me how much it helps performance to defer this traversal to a
> bit later.

... which would be out of the part of codepath that is shared with e.g.
reads and writes on pipes.

> Finally it seems a bit error prone to be checking some IOCB_ flags directly
> while IOCB_DSYNC needs to be checked with iocb_is_dsync() instead. I think
> we'll grow some place mistakenly checking IOCB_DSYNC sooner rather than
> later. So maybe at least rename IOCB_DSYNC to __IOCB_DSYNC to make it more
> obvious in the name that something unusual is going on?

That might make sense...