Re: [PATCH v5 04/13] mm: Add readahead address space operation

Dave Chinner
2020-02-11T20:08:40Z

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