On Mon 24-01-22 13:06:03, John Hubbard wrote: > On 1/24/22 02:05, Jan Kara wrote: > ... > > > 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 > > So it sounds like you prefer this over checking for the zero page in > bio_release_pages(). I'll take a look at both ideas, then, and see what > it looks like. Yes, I somewhat prefer this because it seems more transparent to me. Furthermore if e.g. we can have zero page mapped to userspace (not sure if we can for normal mappings but at least for DAX mapping we can), and userspace provides such mapping as a buffer for direct IO write, then we'll get zero page attached to bio through iov_iter_get_pages() and we'd have to be very careful to special-case zero page in iov_iter_get_pages() as well. Overall it seems fragile to me... So it seems more robust to make sure all pages we attach to bio are pinned. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
