On Tue, Jan 14, 2020 at 06:38:34PM -0800, Matthew Wilcox wrote: > This is an attempt to add a ->readahead op to replace ->readpages. I've > converted two users, iomap/xfs and cifs. The cifs conversion is lacking > fscache support, and that's just because I didn't want to do that work; > I don't believe there's anything fundamental to it. But I wanted to do > iomap because it is The Infrastructure Of The Future and cifs because it > is the sole remaining user of add_to_page_cache_locked(), which enables > the last two patches in the series. By the way, that gives CIFS access > to the workingset shadow infrastructure, which it had to ignore before > because it couldn't put pages onto the lru list at the right time. > > v2: Chris asked me to show what this would look like if we just have > the implementation look up the pages in the page cache, and I managed > to figure out some things I'd done wrong last time. It's even simpler > than v1 (net 104 lines deleted). I want to discuss whether to change the page refcount guarantees while we're changing the API. Currently, page is allocated with a refcount of 1 page is locked, and inserted into page cache and refcount is bumped to 2 ->readahead is called callee is supposed to call put_page() after submitting I/O I/O completion will unlock the page after I/O completes, leaving the refcount at 1. So, what if we leave the refcount at 1 throughout the submission process, saving ourselves two atomic ops per page? We have to ensure that after the page is submitted for I/O, the submission path no longer touches the page. So the process of converting a filesystem to ->readahead becomes slightly more complex, but there's a bugger win as a result. Opinions?
