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Date:   Thu, 23 Mar 2023 07:51:09 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Ritesh Harjani <ritesh.list@gmail.com>,
        Theodore Tso <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 04/31] ext4: Convert ext4_finish_bio() to use folios
Message-ID: <20230323145109.GA466457@frogsfrogsfrogs>
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On Thu, Mar 23, 2023 at 03:26:43AM +0000, Matthew Wilcox wrote:
> On Mon, Mar 06, 2023 at 02:40:55PM +0530, Ritesh Harjani wrote:
> > "Matthew Wilcox (Oracle)" <willy@infradead.org> writes:
> > 
> > > Prepare ext4 to support large folios in the page writeback path.
> > 
> > Sure. I am guessing for ext4 to completely support large folio
> > requires more work like fscrypt bounce page handling doesn't
> > yet support folios right?
> > 
> > Could you please give a little background on what all would be required
> > to add large folio support in ext4 buffered I/O path?
> > (I mean ofcourse other than saying move ext4 to iomap ;))
> > 
> > What I was interested in was, what other components in particular for
> > e.g. fscrypt, fsverity, ext4's xyz component needs large folio support?
> > 
> > And how should one go about in adding this support? So can we move
> > ext4's read path to have large folio support to get started?
> > Have you already identified what all is missing from this path to
> > convert it?
> 
> Honestly, I don't know what else needs to be done beyond this patch
> series.  I can point at some stuff and say "This doesn't work", but in
> general, you have to just enable it and see what breaks.  A lot of the
> buffer_head code is not large-folio safe right now, so that's somewhere
> to go and look.  Or maybe we "just" convert to iomap, and never bother
> fixing the bufferhead code for large folios.

Yes.  Let's leave bufferheads in the legacy doo-doo-dooooo basement
instead of wasting more time on them.  Ideally we'd someday run all the
filesystems through:

bufferheads -> iomap with bufferheads -> iomap with folios -> iomap with
large folios -> retire to somewhere cheaper than Hawaii

--D

> > > Also set the actual error in the mapping, not just -EIO.
> > 
> > Right. I looked at the history and I think it always just had EIO.
> > I think setting the actual err in mapping_set_error() is the right thing
> > to do here.
> > 
> > >
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > 
> > w.r.t this patch series. I reviewed the mechanical changes & error paths
> > which converts ext4 ext4_finish_bio() to use folio.
> > 
> > The changes looks good to me from that perspective. Feel free to add -
> > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> 
> Thanks!