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

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Message-ID: <ZBzy7RHlCqmApxUe@google.com>
Date:   Thu, 23 Mar 2023 17:46:37 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Damien Le Moal <Damien.LeMoal@....com>
Cc:     "chao@...nel.org" <chao@...nel.org>,
        "linux-f2fs-devel@...ts.sourceforge.net" 
        <linux-f2fs-devel@...ts.sourceforge.net>,
        "hans@...tronix.com" <hans@...tronix.com>,
        "hch@...radead.org" <hch@...radead.org>,
        Aravind Ramesh <Aravind.Ramesh@....com>,
        Hans Holmberg <Hans.Holmberg@....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 03/24, Damien Le Moal wrote:
> On Thu, 2023-03-23 at 16:46 -0700, Jaegeuk Kim wrote:
> > On 03/23, Damien Le Moal wrote:
> > > On Thu, 2023-03-23 at 15:14 -0700, Jaegeuk Kim wrote:
> > > > On 03/20, Christoph Hellwig wrote:
> > > > > On Mon, Feb 20, 2023 at 01:20:04PM +0100, Hans Holmberg wrote:
> > > > > > A) Supporting proper direct writes for zoned block devices
> > > > > > would
> > > > > > be the best, but it is currently not supported (probably for
> > > > > > a good but non-obvious reason). Would it be feasible to
> > > > > > implement proper direct IO?
> > > > > 
> > > > > I don't think why not.  In many ways direct writes to zoned
> > > > > devices
> > > > > should be easier than non-direct writes.
> > > > > 
> > > > > Any comments from the maintainers why the direct I/O writes to
> > > > > zoned
> > > > > devices are disabled?  I could not find anything helpful in the
> > > > > comments
> > > > > or commit logs.
> > > > 
> > > > The direct IO wants to overwrite the data on the same block
> > > > address,
> > > > while
> > > > zoned device does not support it?
> > > 
> > > Surely that is not the case with LFS mode, doesn't it ? Otherwise,
> > > even
> > > buffered overwrites would have an issue.
> > 
> > Zoned device only supports LFS mode.
> 
> 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.

> 
> > 
>