[Linux-cachefs] Adventures in NFS re-exporting

Wed Sep 23 21:01:57 UTC 2020

On Wed, Sep 23, 2020 at 09:25:07PM +0100, Daire Byrne wrote:
> 
> ----- On 22 Sep, 2020, at 17:43, Chuck Lever chuck.lever at oracle.com wrote:
> >> On Sep 17, 2020, at 4:23 PM, Frank van der Linden <fllinden at amazon.com> wrote:
> >>
> >> On Thu, Sep 17, 2020 at 03:09:31PM -0400, bfields wrote:
> >>>
> >>> On Thu, Sep 17, 2020 at 05:01:11PM +0100, Daire Byrne wrote:
> >>>>
> >>>> ----- On 15 Sep, 2020, at 18:21, bfields bfields at fieldses.org wrote:
> >>>>
> >>>>>> 4) With an NFSv4 re-export, lots of open/close requests (hundreds per
> >>>>>> second) quickly eat up the CPU on the re-export server and perf top
> >>>>>> shows we are mostly in native_queued_spin_lock_slowpath.
> >>>>>
> >>>>> Any statistics on who's calling that function?
> >>>>
> >>>> I've always struggled to reproduce this with a simple open/close simulation, so
> >>>> I suspect some other operations need to be mixed in too. But I have one
> >>>> production workload that I know has lots of opens & closes (buggy software)
> >>>> included in amongst the usual reads, writes etc.
> >>>>
> >>>> With just 40 clients mounting the reexport server (v5.7.6) using NFSv4.2, we see
> >>>> the CPU of the nfsd threads increase rapidly and by the time we have 100
> >>>> clients, we have maxed out the 32 cores of the server with most of that in
> >>>> native_queued_spin_lock_slowpath.
> >>>
> >>> That sounds a lot like what Frank Van der Linden reported:
> >>>
> >>>        https://lore.kernel.org/linux-nfs/20200608192122.GA19171@dev-dsk-fllinden-2c-c1893d73.us-west-2.amazon.com/
> >>>
> >>> It looks like a bug in the filehandle caching code.
> >>>
> >>> --b.
> >>
> >> Yes, that does look like the same one.
> >>
> >> I still think that not caching v4 files at all may be the best way to go
> >> here, since the intent of the filecache code was to speed up v2/v3 I/O,
> >> where you end up doing a lot of opens/closes, but it doesn't make as
> >> much sense for v4.
> >>
> >> However, short of that, I tested a local patch a few months back, that
> >> I never posted here, so I'll do so now. It just makes v4 opens in to
> >> 'long term' opens, which do not get put on the LRU, since that doesn't
> >> make sense (they are in the hash table, so they are still cached).
> >>
> >> Also, the file caching code seems to walk the LRU a little too often,
> >> but that's another issue - and this change keeps the LRU short, so it's
> >> not a big deal.
> >>
> >> I don't particularly love this patch, but it does keep the LRU short, and
> >> did significantly speed up my testcase (by about 50%). So, maybe you can
> >> give it a try.
> >>
> >> I'll also attach a second patch, that converts the hash table to an rhashtable,
> >> which automatically grows and shrinks in size with usage. That patch also
> >> helped, but not by nearly as much (I think it yielded another 10%).
> >
> > For what it's worth, I applied your two patches to my test server, along
> > with my patch that force-closes cached file descriptors during NFSv4
> > CLOSE processing. The patch combination improves performance (faster
> > elapsed time) for my workload as well.
> 
> I tested Frank's NFSv4 filecache patches with some production workloads and I've hit the below refcount issue a couple of times in the last 48 hours with v5.8.10. This server was re-exporting an NFS client mount at the time.
> 
> Apologies for the spam if I've just hit something unrelated to the patches that is present in v5.8.10.... In truth, I have not used this kernel version before with this workload and just patched it because I had it ready to go. I'll remove the 2 patches and verify.
> 
> Daire
> 
> 
> [ 8930.027838] ------------[ cut here ]------------
> [ 8930.032769] refcount_t: addition on 0; use-after-free.
> [ 8930.038251] WARNING: CPU: 2 PID: 3624 at lib/refcount.c:25 refcount_warn_saturate+0x6e/0xf0
> [ 8930.046799] Modules linked in: tcp_diag inet_diag rpcsec_gss_krb5 nfsv4 dns_resolver act_mirred sch_ingress ifb nfsv3 nfs cls_u32 sch_fq sch_prio cachefiles fscache ext4 mbcache jbd2 sb_edac rapl sg virtio_rng i2c_piix4 input_leds nfsd auth_rpcgss nfs_acl lockd grace binfmt_misc ip_tables xfs libcrc32c sd_mod t10_pi 8021q garp mrp virtio_net net_failover failover virtio_scsi crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel scsi_transport_iscsi crypto_simd cryptd glue_helper virtio_pci virtio_ring virtio serio_raw sunrpc dm_mirror dm_region_hash dm_log dm_mod
> [ 8930.098703] CPU: 2 PID: 3624 Comm: nfsd Tainted: G        W         5.8.10-1.dneg.x86_64 #1
> [ 8930.107391] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> [ 8930.116775] RIP: 0010:refcount_warn_saturate+0x6e/0xf0
> [ 8930.122078] Code: 49 91 18 01 01 e8 57 d6 c2 ff 0f 0b 5d c3 80 3d 38 91 18 01 00 75 d1 48 c7 c7 d0 5c 13 82 c6 05 28 91 18 01 01 e8 37 d6 c2 ff <0f> 0b 5d c3 80 3d 1a 91 18 01 00 75 b1 48 c7 c7 a8 5c 13 82 c6 05
> [ 8930.141107] RSP: 0018:ffffc900012efc70 EFLAGS: 00010282
> [ 8930.146497] RAX: 0000000000000000 RBX: ffff888cc12811e0 RCX: 0000000000000000
> [ 8930.153793] RDX: ffff888d0bca8f20 RSI: ffff888d0bc98d40 RDI: ffff888d0bc98d40
> [ 8930.161087] RBP: ffffc900012efc70 R08: ffff888d0bc98d40 R09: 0000000000000019
> [ 8930.168380] R10: 000000000000072e R11: ffffc900012efad8 R12: ffff888b8bdad600
> [ 8930.175680] R13: ffff888cd428ebe0 R14: ffff8889264f9170 R15: 0000000000000000
> [ 8930.182976] FS:  0000000000000000(0000) GS:ffff888d0bc80000(0000) knlGS:0000000000000000
> [ 8930.191231] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 8930.197139] CR2: 00007fbe43ca1248 CR3: 0000000ce48ee004 CR4: 00000000001606e0
> [ 8930.204436] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 8930.211734] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 8930.219027] Call Trace:
> [ 8930.221665]  nfsd4_process_open2+0xa48/0xec0 [nfsd]
> [ 8930.226724]  ? nfsd_permission+0x6b/0x100 [nfsd]
> [ 8930.231524]  ? fh_verify+0x167/0x210 [nfsd]
> [ 8930.235893]  nfsd4_open+0x407/0x820 [nfsd]
> [ 8930.240248]  nfsd4_proc_compound+0x3c2/0x760 [nfsd]
> [ 8930.245296]  ? nfsd4_decode_compound.constprop.0+0x3a9/0x450 [nfsd]
> [ 8930.251734]  nfsd_dispatch+0xe2/0x220 [nfsd]
> [ 8930.256213]  svc_process_common+0x47b/0x6f0 [sunrpc]
> [ 8930.261355]  ? svc_sock_secure_port+0x16/0x30 [sunrpc]
> [ 8930.266707]  ? nfsd_svc+0x330/0x330 [nfsd]
> [ 8930.270981]  svc_process+0xc5/0x100 [sunrpc]
> [ 8930.275423]  nfsd+0xe8/0x150 [nfsd]
> [ 8930.280028]  kthread+0x114/0x150
> [ 8930.283434]  ? nfsd_destroy+0x60/0x60 [nfsd]
> [ 8930.287875]  ? kthread_park+0x90/0x90
> [ 8930.291700]  ret_from_fork+0x22/0x30
> [ 8930.295447] ---[ end trace c551536c3520545c ]---

It's entirely possible that my patch introduces a refcounting error - it was
intended as a proof-of-concept on how to fix the LRU locking issue for v4
open file caching (while keeping it enabled) - which is why I didn't
"formally" send it in.

Having said that, I don't immediately see the problem.

Maybe try it without the rhashtable patch, that is much less of an
optimization.

The problem would have to be nf_ref as part of nfsd_file, or fi_ref as part
of nfs4_file. If it's the latter, it's probably the rhashtable change.

- Frank