On Wed 22-01-20 10:44:14, Jan Kara wrote: > On Tue 21-01-20 13:48:45, Matthew Wilcox wrote: > > On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote: > > > > 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 have an unfinished patch series laying around that pulls the ->readpage > > > / ->readpages API in somewhat different direction so I'd like to discuss > > > whether it's possible to solve my problem using your API. The problem I > > > have is that currently some operations such as hole punching can race with > > > ->readpage / ->readpages like: > > > > > > CPU0 CPU1 > > > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) > > > filemap_write_and_wait_range() > > > down_write(inode->i_rwsem); > > > truncate_pagecache_range(); > > > readahead(fd, off, len) > > > creates pages in page cache > > > looks up block mapping > > > removes blocks from inode and frees them > > > issues bio > > > - reads stale data - > > > potential security > > > issue > > > > > > Now how I wanted to address this is that I'd change the API convention for > > > ->readpage() so that we call it with the page unlocked and the function > > > would lock the page, check it's still OK, and do what it needs. And this > > > will allow ->readpage() and also ->readpages() to grab lock > > > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching > > > while we are adding pages to page cache and mapping underlying blocks. > > > > > > Now your API makes even ->readpages() (actually ->readahead) called with > > > pages locked so that makes this approach problematic because of lock > > > inversions. So I'd prefer if we could keep the situation that ->readpages / > > > ->readahead gets called without any pages in page cache locked... > > > > I'm not a huge fan of that approach because it increases the number of > > atomic ops (right now, we __SetPageLocked on the page before adding it > > to i_pages). > > Yeah, good point. The per-page cost of locking may be noticeable. Thinking about this a bit more, we should be using ->readpages() to fill most of the data. And for ->readpages() there would be no additional overhead. Just for ->readpage() which should be rarely needed. We just need to come up with a good solution for filesystems that have ->readpage but not ->readpages. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
