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From: Dave Chinner <david@fromorbit.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
	ocfs2-devel@oss.oracle.com, linux-xfs@vger.kernel.org
Subject: Re: [PATCH v5 04/13] mm: Add readahead address space operation
Date: Wed, 12 Feb 2020 07:08:26 +1100
Message-ID: <20200211200826.GK10776@dread.disaster.area> (raw)
In-Reply-To: <20200211125413.GU8731@bombadil.infradead.org>

On Tue, Feb 11, 2020 at 04:54:13AM -0800, Matthew Wilcox wrote:
> On Tue, Feb 11, 2020 at 03:52:30PM +1100, Dave Chinner wrote:
> > > +struct readahead_control {
> > > +	struct file *file;
> > > +	struct address_space *mapping;
> > > +/* private: use the readahead_* accessors instead */
> > > +	pgoff_t start;
> > > +	unsigned int nr_pages;
> > > +	unsigned int batch_count;
> > > +};
> > > +
> > > +static inline struct page *readahead_page(struct readahead_control *rac)
> > > +{
> > > +	struct page *page;
> > > +
> > > +	if (!rac->nr_pages)
> > > +		return NULL;
> > > +
> > > +	page = xa_load(&rac->mapping->i_pages, rac->start);
> > > +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > > +	rac->batch_count = hpage_nr_pages(page);
> > > +	rac->start += rac->batch_count;
> > 
> > There's no mention of large page support in the patch description
> > and I don't recall this sort of large page batching in previous
> > iterations.
> > 
> > This seems like new functionality to me, not directly related to
> > the initial ->readahead API change? What have I missed?
> 
> I had a crisis of confidence when I was working on this -- the loop
> originally looked like this:
> 
> #define readahead_for_each(rac, page)                                   \
>         for (; (page = readahead_page(rac)); rac->nr_pages--)
> 
> and then I started thinking about what I'd need to do to support large
> pages, and that turned into
> 
> #define readahead_for_each(rac, page)                                   \
>         for (; (page = readahead_page(rac));				\
> 		rac->nr_pages -= hpage_nr_pages(page))
> 
> but I realised that was potentially a use-after-free because 'page' has
> certainly had put_page() called on it by then.  I had a brief period
> where I looked at moving put_page() away from being the filesystem's
> responsibility and into the iterator, but that would introduce more
> changes into the patchset, as well as causing problems for filesystems
> that want to break out of the loop.
> 
> By this point, I was also looking at the readahead_for_each_batch()
> iterator that btrfs uses, and so we have the batch count anyway, and we
> might as well use it to store the number of subpages of the large page.
> And so it became easier to just put the whole ball of wax into the initial
> patch set, rather than introduce the iterator now and then fix it up in
> the patch set that I'm basing on this.
> 
> So yes, there's a certain amount of excess functionality in this patch
> set ... I can remove it for the next release.

I'd say "Just document it" as that was the main reason I noticed it.
Or perhaps add the batching function as a stand-alone patch so it's
clear that the batch interface solves two problems at once - large
pages and the btrfs page batching implementation...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply index

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11  1:03 [PATCH v5 00/13] Change readahead API Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 01/13] mm: Fix the return type of __do_page_cache_readahead Matthew Wilcox
2020-02-11  8:19   ` Johannes Thumshirn
2020-02-11 12:34     ` Matthew Wilcox
2020-02-12 18:13   ` Christoph Hellwig
2020-02-14  3:19   ` John Hubbard
2020-02-14  4:21     ` Matthew Wilcox
2020-02-14  4:33       ` John Hubbard
2020-02-14 19:50   ` Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 02/13] mm: Ignore return value of ->readpages Matthew Wilcox
2020-02-12 18:13   ` Christoph Hellwig
2020-02-11  1:03 ` [PATCH v5 03/13] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-02-14  3:36   ` John Hubbard
2020-02-15  1:15     ` Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 04/13] mm: Add readahead address space operation Matthew Wilcox
2020-02-11  4:52   ` Dave Chinner
2020-02-11 12:54     ` Matthew Wilcox
2020-02-11 20:08       ` Dave Chinner [this message]
2020-02-12 18:18   ` Christoph Hellwig
2020-02-14  5:36   ` John Hubbard
2020-02-15  1:15     ` Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 05/13] mm: Add page_cache_readahead_limit Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 06/13] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-02-13 22:09   ` Junxiao Bi
2020-02-11  1:03 ` [PATCH v5 07/13] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 08/13] erofs: Convert uncompressed files " Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 09/13] erofs: Convert compressed " Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 10/13] ext4: Convert " Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 11/13] f2fs: " Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 12/13] fuse: " Matthew Wilcox
2020-02-11  1:03 ` [PATCH v5 13/13] iomap: " Matthew Wilcox

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