linux-kernel - Re: [PATCH] iomap: Ensure iop->uptodate matches PageUptodate

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Message-ID: <20200728092301.GA32142@infradead.org>
Date:   Tue, 28 Jul 2020 10:23:01 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Brian Foster <bfoster@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iomap: Ensure iop->uptodate matches PageUptodate

On Mon, Jul 27, 2020 at 09:53:35AM +1000, Dave Chinner wrote:
> Yes, I understand the code accepts it can happen; what I dislike is
> code that asserts subtle behaviour can happen, then doesn't describe
> that exactly why/how that condition can occur. And then, because we
> don't know exactly how something happens, we add work arounds to
> hide issues we can't reason through fully. That's .... suboptimal.
> 
> Christoph might know off the top of his head how we get into this
> state. Once we work it out, then we need to add comments...

Unfortunately I don't know offhand.  I'll need to spend some more
quality time with this code first.

> > Way ahead of you
> > http://git.infradead.org/users/willy/pagecache.git/commitdiff/5a1de6fc4f815797caa4a2f37c208c67afd7c20b
> 
> *nod*
> 
> I would suggest breaking that out as a separate cleanup patch and
> not hide is in a patch that contains both THP modifications and bug
> fixes. It stands alone as a valid cleanup.

I'm pretty sure I already suggested that when it first showed up.

That being said I have another somewhat related thing in this area
that I really want to get done before THP support, and maybe I can
offload it to willy:

Currently we always allocate the iomap_page structure for blocksize
< PAGE_SIZE.  While this was easy to implement and a major improvement
over the buffer heads it actually is quite silly, as we only actually
need it if we either have sub-page uptodate state, or have extents
boundaries in the page.  So what I'd like to do is to only actually
allocate it in that case.  By doing the allocation lazy it should also
help to never allocate one that is marked all uptodate from the start.