Re: [PATCH RFC 00/30] Overhaul NFSD filecache

From: Chuck Lever III <chuck.lever@oracle.com>
To: Wang Yugui <wangyugui@e16-tech.com>
Cc: Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"david@fromorbit.com" <david@fromorbit.com>,
	"tgraf@suug.ch" <tgraf@suug.ch>, Jeff Layton <jlayton@redhat.com>
Subject: Re: [PATCH RFC 00/30] Overhaul NFSD filecache
Date: Wed, 22 Jun 2022 19:04:39 +0000	[thread overview]
Message-ID: <FE520DC8-3C8F-4974-9F3B-84DE822CB899@oracle.com> (raw)
In-Reply-To: <20220623023645.F914.409509F4@e16-tech.com>

> On Jun 22, 2022, at 2:36 PM, Wang Yugui <wangyugui@e16-tech.com> wrote:
> 
> Hi,
> 
> fstests generic/531 triggered a panic on kernel 5.19.0-rc3 with this
> patchset.

As I mention in the cover letter, I haven't tried running generic/531
yet -- no claim at all that this is finished work and that #386 has
been fixed at this point. I'm merely interested in comments on the
general approach.

> [  405.478056] BUG: kernel NULL pointer dereference, address: 0000000000000049

The "RIP: " tells the location of the crash. Notice that the call
trace here does not include that information. From your attachment:

[  405.518022] RIP: 0010:nfsd_do_file_acquire+0x4e1/0xb80 [nfsd]

To match that to a line of source code:

[cel@manet ~]$ cd src/linux/linux/
[cel@manet linux]$ scripts/faddr2line ../obj/manet/fs/nfsd/filecache.o nfsd_do_file_acquire+0x4e1
nfsd_do_file_acquire+0x4e1/0xfc0:
rht_bucket_insert at /home/cel/src/linux/linux/include/linux/rhashtable.h:303
(inlined by) __rhashtable_insert_fast at /home/cel/src/linux/linux/include/linux/rhashtable.h:718
(inlined by) rhashtable_lookup_get_insert_key at /home/cel/src/linux/linux/include/linux/rhashtable.h:982
(inlined by) nfsd_file_insert at /home/cel/src/linux/linux/fs/nfsd/filecache.c:1031
(inlined by) nfsd_do_file_acquire at /home/cel/src/linux/linux/fs/nfsd/filecache.c:1089
[cel@manet linux]$

This is an example, I'm sure my compiled objects don't match yours.

And, now that I've added observability, you should be able to do:

  # watch cat /proc/fs/nfsd/filecache

to see how many items are in the hash and LRU list while the test
is running.

> [  405.608016] Call Trace:
> [  405.608016]  <TASK>
> [  405.613020]  nfs4_get_vfs_file+0x325/0x410 [nfsd]
> [  405.618018]  nfsd4_process_open2+0x4ba/0x16d0 [nfsd]
> [  405.623016]  ? inode_get_bytes+0x38/0x40
> [  405.623016]  ? nfsd_permission+0x97/0xf0 [nfsd]
> [  405.628022]  ? fh_verify+0x1cc/0x6f0 [nfsd]
> [  405.633025]  nfsd4_open+0x640/0xb30 [nfsd]
> [  405.638025]  nfsd4_proc_compound+0x3bd/0x710 [nfsd]
> [  405.643017]  nfsd_dispatch+0x143/0x270 [nfsd]
> [  405.648019]  svc_process_common+0x3bf/0x5b0 [sunrpc]
> 
> more detail in attachment file(531.dmesg)
> 
> local.config of fstests:
> 	export NFS_MOUNT_OPTIONS="-o rw,relatime,vers=4.2,nconnect=8"
> changes of generic/531
> 	max_allowable_files=$(( 1 * 1024 * 1024 / $nr_cpus / 2 ))

Changed from:

	max_allowable_files=$(( $(cat /proc/sys/fs/file-max) / $nr_cpus / 2 ))

For my own information, what's $nr_cpus in your test?

Aside from the max_allowable_files setting, can you tell how the
test determines when it should stop creating files? Is it looking
for a particular error code from open(2), for instance?

On my client:

[cel@morisot generic]$ cat /proc/sys/fs/file-max
9223372036854775807
[cel@morisot generic]$

I wonder if it's realistic to expect an NFSv4 server to support
that many open files. Is 9 quintillion files really something
I'm going to have to engineer for, or is this just a crazy
test?

> Best Regards
> Wang Yugui (wangyugui@e16-tech.com)
> 2022/06/23
> 
>> This series overhauls the NFSD filecache, a cache of server-side
>> "struct file" objects recently used by NFS clients. The purposes of
>> this overhaul are an immediate improvement in cache scalability in
>> the number of open files, and preparation for further improvements.
>> 
>> There are three categories of patches in this series:
>> 
>> 1. Add observability of cache operation so we can see what we're
>> doing as changes are made to the code.
>> 
>> 2. Improve the scalability of filecache garbage collection,
>> addressing several bugs along the way.
>> 
>> 3. Improve the scalability of the filecache hash table by converting
>> it to use rhashtable.
>> 
>> The series as it stands survives typical test workloads. Running
>> stress-tests like generic/531 is the next step.
>> 
>> These patches are also available in the linux-nfs-bugzilla-386
>> branch of
>> 
>>  https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git 
>> 
>> ---
>> 
>> Chuck Lever (30):
>>      NFSD: Report filecache LRU size
>>      NFSD: Report count of calls to nfsd_file_acquire()
>>      NFSD: Report count of freed filecache items
>>      NFSD: Report average age of filecache items
>>      NFSD: Add nfsd_file_lru_dispose_list() helper
>>      NFSD: Refactor nfsd_file_gc()
>>      NFSD: Refactor nfsd_file_lru_scan()
>>      NFSD: Report the number of items evicted by the LRU walk
>>      NFSD: Record number of flush calls
>>      NFSD: Report filecache item construction failures
>>      NFSD: Zero counters when the filecache is re-initialized
>>      NFSD: Hook up the filecache stat file
>>      NFSD: WARN when freeing an item still linked via nf_lru
>>      NFSD: Trace filecache LRU activity
>>      NFSD: Leave open files out of the filecache LRU
>>      NFSD: Fix the filecache LRU shrinker
>>      NFSD: Never call nfsd_file_gc() in foreground paths
>>      NFSD: No longer record nf_hashval in the trace log
>>      NFSD: Remove lockdep assertion from unhash_and_release_locked()
>>      NFSD: nfsd_file_unhash can compute hashval from nf->nf_inode
>>      NFSD: Refactor __nfsd_file_close_inode()
>>      NFSD: nfsd_file_hash_remove can compute hashval
>>      NFSD: Remove nfsd_file::nf_hashval
>>      NFSD: Remove stale comment from nfsd_file_acquire()
>>      NFSD: Clean up "open file" case in nfsd_file_acquire()
>>      NFSD: Document nfsd_file_cache_purge() API contract
>>      NFSD: Replace the "init once" mechanism
>>      NFSD: Set up an rhashtable for the filecache
>>      NFSD: Convert the filecache to use rhashtable
>>      NFSD: Clean up unusued code after rhashtable conversion
>> 
>> 
>> fs/nfsd/filecache.c | 677 +++++++++++++++++++++++++++-----------------
>> fs/nfsd/filecache.h |   6 +-
>> fs/nfsd/nfsctl.c    |  10 +
>> fs/nfsd/trace.h     | 117 ++++++--
>> 4 files changed, 522 insertions(+), 288 deletions(-)
>> 
>> --
>> Chuck Lever
> 
> <531.dmesg>

--
Chuck Lever