Date:   Mon, 12 Jun 2017 10:37:27 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Jan Kara <jack@...e.com>, Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [RFC PATCH] fs: ext4: don't trap kswapd and allocating tasks on
 ext4 inode IO

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@...xchg.org>
> > > > 
> > > > 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?

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