From: Qian Cai <cai@lca.pw>
To: Michal Hocko <mhocko@kernel.org>
Cc: David Hildenbrand <david@redhat.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>,
Oscar Salvador <osalvador@suse.de>,
Pavel Tatashin <pasha.tatashin@soleen.com>,
Dan Williams <dan.j.williams@intel.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock
Date: Wed, 25 Sep 2019 14:20:59 -0400 [thread overview]
Message-ID: <1569435659.5576.227.camel@lca.pw> (raw)
In-Reply-To: <20190925174809.GM23050@dhcp22.suse.cz>
On Wed, 2019-09-25 at 19:48 +0200, Michal Hocko wrote:
> On Wed 25-09-19 12:01:02, Qian Cai wrote:
> > On Wed, 2019-09-25 at 09:02 +0200, David Hildenbrand wrote:
> > > On 24.09.19 20:54, Qian Cai wrote:
> > > > On Tue, 2019-09-24 at 17:11 +0200, Michal Hocko wrote:
> > > > > On Tue 24-09-19 11:03:21, Qian Cai wrote:
> > > > > [...]
> > > > > > While at it, it might be a good time to rethink the whole locking over there, as
> > > > > > it right now read files under /sys/kernel/slab/ could trigger a possible
> > > > > > deadlock anyway.
> > > > > >
> > > > >
> > > > > [...]
> > > > > > [ 442.452090][ T5224] -> #0 (mem_hotplug_lock.rw_sem){++++}:
> > > > > > [ 442.459748][ T5224] validate_chain+0xd10/0x2bcc
> > > > > > [ 442.464883][ T5224] __lock_acquire+0x7f4/0xb8c
> > > > > > [ 442.469930][ T5224] lock_acquire+0x31c/0x360
> > > > > > [ 442.474803][ T5224] get_online_mems+0x54/0x150
> > > > > > [ 442.479850][ T5224] show_slab_objects+0x94/0x3a8
> > > > > > [ 442.485072][ T5224] total_objects_show+0x28/0x34
> > > > > > [ 442.490292][ T5224] slab_attr_show+0x38/0x54
> > > > > > [ 442.495166][ T5224] sysfs_kf_seq_show+0x198/0x2d4
> > > > > > [ 442.500473][ T5224] kernfs_seq_show+0xa4/0xcc
> > > > > > [ 442.505433][ T5224] seq_read+0x30c/0x8a8
> > > > > > [ 442.509958][ T5224] kernfs_fop_read+0xa8/0x314
> > > > > > [ 442.515007][ T5224] __vfs_read+0x88/0x20c
> > > > > > [ 442.519620][ T5224] vfs_read+0xd8/0x10c
> > > > > > [ 442.524060][ T5224] ksys_read+0xb0/0x120
> > > > > > [ 442.528586][ T5224] __arm64_sys_read+0x54/0x88
> > > > > > [ 442.533634][ T5224] el0_svc_handler+0x170/0x240
> > > > > > [ 442.538768][ T5224] el0_svc+0x8/0xc
> > > > >
> > > > > I believe the lock is not really needed here. We do not deallocated
> > > > > pgdat of a hotremoved node nor destroy the slab state because an
> > > > > existing slabs would prevent hotremove to continue in the first place.
> > > > >
> > > > > There are likely details to be checked of course but the lock just seems
> > > > > bogus.
> > > >
> > > > Check 03afc0e25f7f ("slab: get_online_mems for
> > > > kmem_cache_{create,destroy,shrink}"). It actually talk about the races during
> > > > memory as well cpu hotplug, so it might even that cpu_hotplug_lock removal is
> > > > problematic?
> > > >
> > >
> > > Which removal are you referring to? get_online_mems() does not mess with
> > > the cpu hotplug lock (and therefore this patch).
> >
> > The one in your patch. I suspect there might be races among the whole NUMA node
> > hotplug, kmem_cache_create, and show_slab_objects(). See bfc8c90139eb ("mem-
> > hotplug: implement get/put_online_mems")
> >
> > "kmem_cache_{create,destroy,shrink} need to get a stable value of cpu/node
> > online mask, because they init/destroy/access per-cpu/node kmem_cache parts,
> > which can be allocated or destroyed on cpu/mem hotplug."
>
> I still have to grasp that code but if the slub allocator really needs
> a stable cpu mask then it should be using the explicit cpu hotplug
> locking rather than rely on side effect of memory hotplug locking.
>
> > Both online_pages() and show_slab_objects() need to get a stable value of
> > cpu/node online mask.
>
> Could tou be more specific why online_pages need a stable cpu online
> mask? I do not think that show_slab_objects is a real problem because a
> potential race shouldn't be critical.
build_all_zonelists()
__build_all_zonelists()
for_each_online_cpu(cpu)
next prev parent reply other threads:[~2019-09-25 18:21 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-24 14:36 [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock David Hildenbrand
2019-09-24 14:48 ` Michal Hocko
2019-09-24 15:01 ` David Hildenbrand
2019-09-24 15:03 ` Qian Cai
2019-09-24 15:11 ` Michal Hocko
2019-09-24 18:54 ` Qian Cai
2019-09-25 7:02 ` David Hildenbrand
2019-09-25 16:01 ` Qian Cai
2019-09-25 17:48 ` Michal Hocko
2019-09-25 18:20 ` Qian Cai [this message]
2019-09-25 19:48 ` David Hildenbrand
2019-09-25 20:32 ` Qian Cai
2019-09-26 7:26 ` David Hildenbrand
2019-09-26 7:38 ` Michal Hocko
2019-09-26 7:26 ` Michal Hocko
2019-09-26 11:19 ` Qian Cai
2019-09-26 11:52 ` Michal Hocko
2019-09-26 13:02 ` Qian Cai
2019-09-26 13:14 ` David Hildenbrand
2019-09-25 10:03 ` Michal Hocko
2019-09-24 15:23 ` David Hildenbrand
2019-10-02 21:37 ` Qian Cai
2019-10-04 7:42 ` David Hildenbrand
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=1569435659.5576.227.camel@lca.pw \
--to=cai@lca.pw \
--cc=akpm@linux-foundation.org \
--cc=dan.j.williams@intel.com \
--cc=david@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@kernel.org \
--cc=osalvador@suse.de \
--cc=pasha.tatashin@soleen.com \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).