Re: [RFC v3 1/7] fs: Add folio_may_straddle_isize helper

From: Christoph Hellwig <hch@infradead.org>
To: "Andreas Grünbacher" <andreas.gruenbacher@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	"Darrick J . Wong" <djwong@kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Matthew Wilcox <willy@infradead.org>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-ext4@vger.kernel.org, cluster-devel@redhat.com
Subject: Re: [RFC v3 1/7] fs: Add folio_may_straddle_isize helper
Date: Fri, 23 Dec 2022 23:21:24 -0800	[thread overview]
Message-ID: <Y6aodOf7Q016xSay@infradead.org> (raw)
In-Reply-To: <CAHpGcMKJO7HhgyU5NKX3h6vVeNAGp-8xFrOf+nSTEWHC-PekzA@mail.gmail.com>

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).