linux-ext4 - Re: [PATCH v7 22/24] iomap: Convert from readpages to readahead

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Message-ID: <20200222010013.GH9506@magnolia>
Date:   Fri, 21 Feb 2020 17:00:13 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        ocfs2-devel@....oracle.com, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v7 22/24] iomap: Convert from readpages to readahead

On Thu, Feb 20, 2020 at 08:57:34AM -0800, Matthew Wilcox wrote:
> On Thu, Feb 20, 2020 at 07:49:12AM -0800, Christoph Hellwig wrote:
> > > +/**
> > > + * iomap_readahead - Attempt to read pages from a file.
> > > + * @rac: Describes the pages to be read.
> > > + * @ops: The operations vector for the filesystem.
> > > + *
> > > + * This function is for filesystems to call to implement their readahead
> > > + * address_space operation.
> > > + *
> > > + * Context: The file is pinned by the caller, and the pages to be read are
> > > + * all locked and have an elevated refcount.  This function will unlock
> > > + * the pages (once I/O has completed on them, or I/O has been determined to
> > > + * not be necessary).  It will also decrease the refcount once the pages
> > > + * have been submitted for I/O.  After this point, the page may be removed
> > > + * from the page cache, and should not be referenced.
> > > + */
> > 
> > Isn't the context documentation something that belongs into the aop
> > documentation?  I've never really seen the value of duplicating this
> > information in method instances, as it is just bound to be out of date
> > rather sooner than later.
> 
> I'm in two minds about it as well.  There's definitely no value in
> providing kernel-doc for implementations of a common interface ... so
> rather than fixing the nilfs2 kernel-doc, I just deleted it.  But this
> isn't just the implementation, like nilfs2_readahead() is, it's a library
> function for filesystems to call, so it deserves documentation.  On the
> other hand, there's no real thought to this on the part of the filesystem;
> the implementation just calls this with the appropriate ops pointer.
> 
> Then again, I kind of feel like we need more documentation of iomap to
> help filesystems convert to using it.  But maybe kernel-doc isn't the
> mechanism to provide that.

I think we need more documentation of the parts of iomap where it can
call back into the filesystem (looking at you, iomap_dio_ops).

I'm not opposed to letting this comment stay, though I don't see it as
all that necessary since iomap_readahead implements a callout that's
documented in vfs.rst and is thus subject to all the constraints listed
in the (*readahead) documentation.

--D