On Tue 31-05-11 18:27:20, Ted Tso wrote: > On Tue, May 31, 2011 at 01:22:53PM +0200, Jan Kara wrote: > > > > The problem is that with ext4, we need i_mutex in io completion path to > > end page writeback. So we cannot do GFP_KERNEL allocation whenever we hold > > i_mutex because mm might wait in direct reclaim for IO to complete and that > > cannot happen until we release i_mutex. > > OK, maybe I'm being dense, but I'm not seeing it. I see where we need > i_mutex on the ext4_da_writepages() codepath, but that's never used > for direct reclaim. Direct reclaim only calls ext4_writepage(), and > that doesn't seem to try to grab i_mutex as near as I can tell. Am I > missing something? What happens is that direct reclaim sometimes does wait_on_page_writeback() (e.g. shrink_page_list()) or it explicitely waits for NR_WRITEBACK statistics to go below some threshold (throttle_vm_writeout()). And that is deadlockable if we hold i_mutex while doing this because we may need i_mutex to actually move the page from PageWriteback state... As I'm saying this, I've realized ext4 has this problem also with stable-pages patches because there we can wait for PageWriteback in grab_cache_page_write_begin() when we also hold i_mutex. So I think we'll have to come up with a way to convert unwritten extents without having to hold i_mutex. That's going to be interesting. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
