Dave Chinner
2020-Feb-11 20:08 UTC
[Ocfs2-devel] [PATCH v5 04/13] mm: Add readahead address space operation
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 at fromorbit.com