On 8/31/20 10:56 AM, Matthew Wilcox wrote: > On Mon, Aug 31, 2020 at 10:39:26AM -0600, Jens Axboe wrote: >> We really should ensure that ->io_pages is always set, imho, instead of >> having to work-around it in other spots. > > Interestingly, there are only three places in the entire kernel which > _use_ bdi->io_pages. FAT, Verity and the pagecache readahead code. > > Verity: > unsigned long num_ra_pages = > min_t(unsigned long, num_blocks_to_hash - i, > inode->i_sb->s_bdi->io_pages); > > FAT: > if (ra_pages > sb->s_bdi->io_pages) > ra_pages = rounddown(ra_pages, sb->s_bdi->io_pages); > > Pagecache: > max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages); > and > if (req_size > max_pages && bdi->io_pages > max_pages) > max_pages = min(req_size, bdi->io_pages); > > The funny thing is that all three are using it differently. Verity is > taking io_pages to be the maximum amount to readahead. FAT is using > it as the unit of readahead (round down to the previous multiple) and > the pagecache uses it to limit reads that exceed the current per-file > readahead limit (but allows per-file readahead to exceed io_pages, > in which case it has no effect). > > So how should it be used? My inclination is to say that the pagecache > is right, by virtue of being the most-used. When I added ->io_pages, it was for the page cache use case. The others grew after that... -- Jens Axboe
