On Wed, Jun 24, 2020 at 2:35 PM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote: > On Mon, Jun 22, 2020 at 8:13 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Mon, Jun 22, 2020 at 04:35:05PM +0200, Andreas Gruenbacher wrote: > > > I'm fine with not moving that functionality into the VFS. The problem > > > I have in gfs2 is that taking glocks is really expensive. Part of that > > > overhead is accidental, but we definitely won't be able to fix it in > > > the short term. So something like the IOCB_CACHED flag that prevents > > > generic_file_read_iter from issuing readahead I/O would save the day > > > for us. Does that idea stand a chance? > > > > For the short-term fix, is switching to a trylock in gfs2_readahead() > > acceptable? > > Well, it's the only thing we can do for now, right? It turns out that gfs2 can still deadlock with a trylock in gfs2_readahead, just differently: in this instance, gfs2_glock_nq will call inode_dio_wait. When there is pending direct I/O, we'll end up waiting for iomap_dio_complete, which will call invalidate_inode_pages2_range, which will try to lock the pages already locked for gfs2_readahead. This late in the 5.8 release cycle, I'd like to propose converting gfs2 back to use mpage_readpages. This requires reinstating mpage_readpages, but it's otherwise relatively trivial. We can then introduce an IOCB_CACHED or equivalent flag, fix the locking order in gfs2, convert gfs2 to mpage_readahead, and finally remove mage_readpages in 5.9. I'll post a patch queue that does this for comment. Thanks, Andreas
