linux-ext4 - Re: [PATCH v7 14/24] btrfs: Convert from readpages to readahead

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Date:   Mon, 24 Feb 2020 13:43:47 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-erofs@...ts.ozlabs.org" <linux-erofs@...ts.ozlabs.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-f2fs-devel@...ts.sourceforge.net" 
        <linux-f2fs-devel@...ts.sourceforge.net>,
        "cluster-devel@...hat.com" <cluster-devel@...hat.com>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v7 14/24] btrfs: Convert from readpages to readahead

On Thu, Feb 20, 2020 at 07:57:27AM -0800, Christoph Hellwig wrote:
> On Thu, Feb 20, 2020 at 07:54:52AM -0800, Matthew Wilcox wrote:
> > On Thu, Feb 20, 2020 at 07:46:58AM -0800, Christoph Hellwig wrote:
> > > On Thu, Feb 20, 2020 at 05:48:49AM -0800, Matthew Wilcox wrote:
> > > > btrfs: Convert from readpages to readahead
> > > >   
> > > > Implement the new readahead method in btrfs.  Add a readahead_page_batch()
> > > > to optimise fetching a batch of pages at once.
> > > 
> > > Shouldn't this readahead_page_batch heper go into a separate patch so
> > > that it clearly stands out?
> > 
> > I'll move it into 'Put readahead pages in cache earlier' for v8 (the
> > same patch where we add readahead_page())
> 
> One argument for keeping it in a patch of its own is that btrfs appears
> to be the only user, and Goldwyn has a WIP conversion of btrfs to iomap,
> so it might go away pretty soon and we could just revert the commit.
> 
> But this starts to get into really minor details, so I'll shut up now :)

So looking at this again I have another comment and a question.

First I think the implicit ARRAY_SIZE in readahead_page_batch is highly
dangerous, as it will do the wrong thing when passing a pointer or
function argument.

Second I wonder іf it would be worth to also switch to a batched
operation in iomap if the xarray overhead is high enough.  That should
be pretty trivial, but we don't really need to do it in this series.