linux-ext4 - Re: [RFC v3 1/7] fs: Add folio_may_straddle

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Date:   Fri, 23 Dec 2022 23:21:24 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com
Subject: Re: [RFC v3 1/7] fs: Add folio_may_straddle_isize helper

On Fri, Dec 23, 2022 at 11:04:51PM +0100, Andreas Grünbacher wrote:
> > I find the naming very confusing.  Any good reason to not follow
> > the naming of pagecache_isize_extended an call it
> > folio_isize_extended?
> 
> A good reason for a different name is because
> folio_may_straddle_isize() requires a locked folio, while
> pagecache_isize_extended() will fail if the folio is still locked. So
> this doesn't follow the usual "replace 'page' with 'folio'" pattern.

pagecache also doesn't say page, it says pagecache.  I'd still prepfer
to keep the postfix the same.  And I think the fact that it needs
a locked folio should also have an assert, which both documents this
and catches errors.  I think that's much better than an arbitrarily
different name.

> > Should pagecache_isize_extended be rewritten to use this helper,
> > i.e. turn this into a factoring out of a helper?
> 
> I'm not really sure about that. The boundary conditions in the two
> functions are not identical. I think the logic in
> folio_may_straddle_isize() is sufficient for the
> extending-write-under-folio-lock case, but I'd still need confirmation
> for that. If the same logic would also be enough in
> pagecache_isize_extended() is more unclear to me.

That's another thing that really needs to into the commit log,
why is the condition different and pagecache_isize_extended can't
just be extended for it (if it really can't).