linux-kernel - Re: [RFC PATCH] f2fs: preserve direct write semantics when buffering is forced

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Message-ID: <ZH45N4nsqxCnhnoU@google.com>
Date:   Mon, 5 Jun 2023 12:36:23 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Hans Holmberg <Hans.Holmberg@....com>
Cc:     "hch@...radead.org" <hch@...radead.org>,
        Damien Le Moal <Damien.LeMoal@....com>,
        "chao@...nel.org" <chao@...nel.org>,
        "linux-f2fs-devel@...ts.sourceforge.net" 
        <linux-f2fs-devel@...ts.sourceforge.net>,
        "hans@...tronix.com" <hans@...tronix.com>,
        Aravind Ramesh <Aravind.Ramesh@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] f2fs: preserve direct write semantics when buffering
 is forced

On 06/05, Hans Holmberg wrote:
> 
> On Sun, Mar 26, 2023 at 04:39:10PM -0700, hch@...radead.org wrote:
> > On Thu, Mar 23, 2023 at 05:46:37PM -0700, Jaegeuk Kim wrote:
> > > > Yes, and that was exactly my point: with LFS mode, O_DIRECT write
> > > > should never overwrite anything. So I do not see why direct writes
> > > > should be handled as buffered writes with zoned devices. Am I missing
> > > > something here ?
> > > 
> > > That's an easiest way to serialize block allocation and submit_bio when users
> > > are calling buffered writes and direct writes in parallel. :)
> > > I just felt that if we can manage both of them in direct write path along with
> > > buffered write path, we may be able to avoid memcpy.
> > 
> > Yes.  Note that right now f2fs doesn't really support proper O_DIRECT
> > for buffered I/O either, as non-overwrites require a feature similar
> > to unwritten extents, or a split of the allocation phase and the record
> > metdata phase.  If we'd go for the second choice for f2fs, which is the
> > more elegant thing to do, you'll get the zoned direct I/O write support
> > almost for free.
> 
> So, Jaegeuk, do you think suporting direct io proper is the way to do to fix this
> issue? That looks like a better solution to me (at least long term).
> 
> Until that would be put into place, do you want my fix (with your code
> style fixes) rebased and resent?

Yes, it's already landed in 6.4-rc1 of Linus tree, and surely I have the topic
in my long term plan.

Thanks,

> 
> Cheers,
> Hans