On Mon, Jun 14, 2010 at 09:37:32PM -0700, Andrew Morton wrote: > On Tue, 15 Jun 2010 00:08:14 -0400 Rik van Riel <riel@xxxxxxxxxx> wrote: > > > On 06/14/2010 09:45 PM, Andrew Morton wrote: > > > On Mon, 14 Jun 2010 21:16:29 -0400 Rik van Riel<riel@xxxxxxxxxx> wrote: > > > > > >> Would it be hard to add a "please flush this file" > > >> way to call the filesystem flushing threads? > > > > > > Passing the igrab()bed inode into the flusher threads would fix the > > > iput_final() problems, as long as the alloc_pages() caller never blocks > > > indefinitely waiting for the work which the flusher threads are doing. > > > > > > Otherwise we get (very hard-to-hit) deadlocks where the alloc_pages() > > > caller holds VFS locks and is waiting for the flusher threads while all > > > the flusher threads are stuck under iput_final() waiting for those VFS > > > locks. > > > > > > That's fixable by not using igrab()/iput(). You can use lock_page() to > > > pin the address_space. Pass the address of the locked page across to > > > the flusher threads so they don't try to lock it a second time, or just > > > use trylocking on that writeback path or whatever. > > > > Any thread that does not have __GFP_FS set in its gfp_mask > > cannot wait for the flusher to complete. This is regardless > > of the mechanism used to kick the flusher. > > mm... kinda. A bare order-zero __GFP_WAIT allocation can still wait > forever, afaict. > > > Then again, those threads cannot call ->writepage today > > either, so we should be fine keeping that behaviour. > > I'm not sure. iput_final() can take a lot of locks, both VFS and > heaven knows what within the individual filesystems. Is it the case > that all allocations which occur under all of those locks is always > !__GFP_FS? Hard to say... __GFP_FS is set with i_mutex held in places, and there is nothing to prevent a filesystem from using that in iput_final paths, AFAIK. > > Threads that do have __GFP_FS in their gfp_mask can wait > > for the flusher in various ways. Maybe the lock_page() > > method can be simplified by having the flusher thread > > unlock the page the moment it gets it, and then run the > > normal flusher code? > > Well, _something_ has to pin the address_space. A single locked page > will do. > > > The pageout code (in shrink_page_list) already unlocks > > the page anyway before putting it back on the relevant > > LRU list. It would be easy enough to skip that unlock > > and let the flusher thread take care of it. > > Once that page is unlocked, we can't touch *mapping - its inode can be > concurrently reclaimed. Although I guess the technique in > handle_write_error() can be reused. Nasty. That guy needs to be using lock_page_nosync(). -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
