Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

From: Andrew Morton <akpm@linux-foundation.org>
To: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jens Axboe <jaxboe@fusionio.com>, Chris Mason <clm@fb.com>,
	Josef Bacik <jbacik@fb.com>, LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Neil Brown <neilb@suse.de>, Christoph Hellwig <hch@lst.de>,
	Tejun Heo <tj@kernel.org>,
	linux-mm@kvack.org
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()
Date: Mon, 21 Sep 2015 13:21:08 -0700	[thread overview]
Message-ID: <20150921132108.990b3ce5e1acd0a7c7e73053@linux-foundation.org> (raw)
In-Reply-To: <20150921092429.GB9028@quack.suse.cz>

On Mon, 21 Sep 2015 11:24:29 +0200 Jan Kara <jack@suse.cz> wrote:

> On Sat 19-09-15 08:17:14, Dave Chinner wrote:
> > On Thu, Sep 17, 2015 at 11:04:03PM -0700, Linus Torvalds wrote:
> > > On Thu, Sep 17, 2015 at 10:40 PM, Dave Chinner <david@fromorbit.com> wrote:
> > > > PS: just hit another "did this just get broken in 4.3-rc1" issue - I
> > > > can't run blktrace while there's a IO load because:
> > > >
> > > > $ sudo blktrace -d /dev/vdc
> > > > BLKTRACESETUP(2) /dev/vdc failed: 5/Input/output error
> > > > Thread 1 failed open /sys/kernel/debug/block/(null)/trace1: 2/No such file or directory
> > > > ....
> > > >
> > > > [  641.424618] blktrace: page allocation failure: order:5, mode:0x2040d0
> > > > [  641.438933]  [<ffffffff811c1569>] kmem_cache_alloc_trace+0x129/0x400
> > > > [  641.440240]  [<ffffffff811424f8>] relay_open+0x68/0x2c0
> > > > [  641.441299]  [<ffffffff8115deb1>] do_blk_trace_setup+0x191/0x2d0
> > > >
> > > > gdb) l *(relay_open+0x68)
> > > > 0xffffffff811424f8 is in relay_open (kernel/relay.c:582).
> > > > 577                     return NULL;
> > > > 578             if (subbuf_size > UINT_MAX / n_subbufs)
> > > > 579                     return NULL;
> > > > 580
> > > > 581             chan = kzalloc(sizeof(struct rchan), GFP_KERNEL);
> > > > 582             if (!chan)
> > > > 583                     return NULL;
> > > > 584
> > > > 585             chan->version = RELAYFS_CHANNEL_VERSION;
> > > > 586             chan->n_subbufs = n_subbufs;
> > > >
> > > > and struct rchan has a member struct rchan_buf *buf[NR_CPUS];
> > > > and CONFIG_NR_CPUS=8192, hence the attempt at an order 5 allocation
> > > > that fails here....
> > > 
> > > Hm. Have you always had MAX_SMP (and the NR_CPU==8192 that it causes)?
> > > From a quick check, none of this code seems to be new.
> > 
> > Yes, I always build MAX_SMP kernels for testing, because XFS is
> > often used on such machines and so I want to find issues exactly
> > like this in my testing rather than on customer machines... :/
> > 
> > > That said, having that
> > > 
> > >         struct rchan_buf *buf[NR_CPUS];
> > > 
> > > in "struct rchan" really is something we should fix. We really should
> > > strive to not allocate things by CONFIG_NR_CPU's, but by the actual
> > > real CPU count.
> > 
> > *nod*. But it doesn't fix the problem of the memory allocation
> > failing when there's still gigabytes of immediately reclaimable
> > memory available in the page cache. If this is failing under page
> > cache memory pressure, then we're going to be doing an awful lot
> > more falling back to vmalloc in the filesystem code where large
> > allocations like this are done e.g. extended attribute buffers are
> > order-5, and used a lot when doing things like backups which tend to
> > also produce significant page cache memory pressure.
> > 
> > Hence I'm tending towards there being a memory reclaim behaviour
> > regression, not so much worrying about whether this specific
> > allocation is optimal or not.
> 
> Yup, looks like a regression in reclaim. Added linux-mm folks to CC.

That's going to be hard to find.  Possibly Vlastimil's 5-patch series
"mm, compaction: more robust check for scanners meeting", possibly
Joonsoo's "mm/compaction: correct to flush migrated pages if pageblock
skip happens".  But probably something else :(

Teach relay.c about alloc_percpu()?