Hello, On Sun 23-01-22 23:52:07, John Hubbard wrote: > Background: despite having very little experience in the block and bio > layers, I am attempting to convert the Direct IO parts of them from > using get_user_pages_fast(), to pin_user_pages_fast(). This requires the > use of a corresponding special release call: unpin_user_pages(), instead > of the generic put_page(). > > Fortunately, Christoph Hellwig has observed [1] (more than once [2]) that > only "a little" refactoring is required, because it is *almost* true > that bio_release_pages() could just be switched over from calling > put_page(), to unpin_user_page(). The "not quite" part is mainly due to > the zero page. There are a few write paths that pad zeroes, and they use > the zero page. > > That's where I'd like some advice. How to refactor things, so that the > zero page does not end up in the list of pages that bio_release_pages() > acts upon? > > To ground this in reality, one of the partial call stacks is: > > do_direct_IO() > dio_zero_block() > page = ZERO_PAGE(0); <-- This is a problem > > I'm not sure what to use, instead of that zero page! The zero page > doesn't need to be allocated nor tracked, and so any replacement > approaches would need either other storage, or some horrid scheme that I > won't go so far as to write on the screen. :) Well, I'm not sure if you consider this ugly but currently we use get_page() in that path exactly so that bio_release_pages() does not have to care about zero page. So now we could grab pin on the zero page instead through try_grab_page() or something like that... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
