Hi Jan, On Mon, Jun 12, 2017 at 10:09:57AM +0200, Jan Kara wrote: > On Tue 16-05-17 18:03:37, Jan Kara wrote: > > On Tue 16-05-17 11:41:05, Johannes Weiner wrote: > > > On Tue, May 16, 2017 at 04:36:45PM +0200, Jan Kara wrote: > > > > On Mon 15-05-17 11:46:34, Johannes Weiner wrote: > > > > > We have observed across several workloads situations where kswapd and > > > > > direct reclaimers get stuck in the inode shrinker of the ext4 / mount, > > > > > causing allocation latencies across tasks in the system, while there > > > > > are dozens of gigabytes of clean page cache covering multiple disks. > > > > > > > > > > The stack traces of such an instance looks like this: > > > > > > > > > > [<ffffffff812b3225>] jbd2_log_wait_commit+0x95/0x110 > > > > > [<ffffffff812b4f29>] jbd2_complete_transaction+0x59/0x90 > > > > > [<ffffffff812668da>] ext4_evict_inode+0x2da/0x480 > > > > > [<ffffffff811f2230>] evict+0xc0/0x190 > > > > > [<ffffffff811f2339>] dispose_list+0x39/0x50 > > > > > [<ffffffff811f323b>] prune_icache_sb+0x4b/0x60 > > > > > [<ffffffff811dba71>] super_cache_scan+0x141/0x190 > > > > > [<ffffffff8116e755>] shrink_slab+0x235/0x440 > > > > > [<ffffffff81172b48>] shrink_zone+0x268/0x2d0 > > > > > [<ffffffff81172f04>] do_try_to_free_pages+0x164/0x410 > > > > > [<ffffffff81173265>] try_to_free_pages+0xb5/0x160 > > > > > [<ffffffff811656b6>] __alloc_pages_nodemask+0x636/0xb30 > > > > > [<ffffffff811acac8>] alloc_pages_current+0x88/0x120 > > > > > [<ffffffff816d4e46>] skb_page_frag_refill+0xc6/0xf0 > > > > > [<ffffffff816d4e8d>] sk_page_frag_refill+0x1d/0x80 > > > > > [<ffffffff8173f86b>] tcp_sendmsg+0x28b/0xb10 > > > > > [<ffffffff81769727>] inet_sendmsg+0x67/0xa0 > > > > > [<ffffffff816d0488>] sock_sendmsg+0x38/0x50 > > > > > [<ffffffff816d0518>] sock_write_iter+0x78/0xd0 > > > > > [<ffffffff811d774e>] do_iter_readv_writev+0x5e/0xa0 > > > > > [<ffffffff811d8468>] do_readv_writev+0x178/0x210 > > > > > [<ffffffff811d871c>] vfs_writev+0x3c/0x50 > > > > > [<ffffffff811d8782>] do_writev+0x52/0xd0 > > > > > [<ffffffff811d9810>] SyS_writev+0x10/0x20 > > > > > [<ffffffff81002910>] do_syscall_64+0x50/0xa0 > > > > > [<ffffffff817eed3c>] return_from_SYSCALL_64+0x0/0x6a > > > > > [<ffffffffffffffff>] 0xffffffffffffffff > > > > > > > > > > The inode shrinker has provisions to skip any inodes that require > > > > > writeback, to avoid tarpitting the entire system behind a single > > > > > object when there are many other pools to recycle memory from. But > > > > > that logic doesn't cover the situation where an ext4 inode is clean > > > > > but journaled and tied to a commit that yet needs to hit the platter. > > > > > > > > > > Add a superblock operation that lets the generic inode shrinker query > > > > > the filesystem whether evicting a given inode will require any IO; add > > > > > an ext4 implementation that checks whether the journal is caught up to > > > > > the commit id associated with the inode. > > > > > > > > > > Fixes: 2d859db3e4a8 ("ext4: fix data corruption in inodes with journalled data") > > > > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > > > > > > > OK. I have to say I'm somewhat surprised you use data journalling on some > > > > of your files / filesystems but whatever - maybe these are long symlink > > > > after all which would make sense. > > > > > > The filesystem is actually mounted data=ordered and we didn't catch > > > anyone in userspace enabling journaling on individual inodes. So we > > > assumed this must be from symlinks. > > > > OK. > > > > > > And I'm actually doubly surprised you can see these stack traces as > > > > these days inode_lru_isolate() checks inode->i_data.nrpages and > > > > uncommitted pages cannot be evicted from pagecache > > > > (ext4_releasepage() will refuse to free them) so I don't see how > > > > such inode can get to dispose_list(). But maybe the inode doesn't > > > > really have any pages and i_datasync_tid just happens to be set to > > > > the current transaction because it is initialized that way and we > > > > are evicting inode that was recently read from disk. > > > > > > Hm, we're running 4.6, but that already has the nrpages check in > > > inode_lru_isolate(). There couldn't be any pages in those inodes by > > > the time the shrinker gets to them. > > > > > > > Anyway if you add: "&& inode->i_data.nrpages" to the test in > > > > ext4_evict_inode() do the stalls go away? > > > > > > Want me to still test this? > > > > Can you try attached patch? I'd like to confirm the theory before merging > > this... Thanks! > > Ping? Any result with this patch? Sorry for not getting back earlier. I was waiting for the internal reporter of this issue to test it, but they have since mitigated the issue by reducing the memory footprint of their application; bad memory health caused other problems as well. Since this is in a production environment, they're reluctant to muck with it now that things are working. However, with or without a reproducer, it seems to make sense to not serialize evict() on commits when we don't have to from a correctness point of view. Would you be opposed to just merging the patch anyway?