Re: [PATCH RFC 0/8] dcache: increase poison resistance

Junxiao Bi
2020-12-09T23:01:31Z

Hi Konstantin,

We tested this patch set recently and found it limiting negative dentry
to a small part of total memory. And also we don't see any performance
regression on it. Do you have any plan to integrate it into mainline? It
will help a lot on memory fragmentation issue causing by dentry slab,
there were a lot of customer cases where sys% was very high since most
cpu were doing memory compaction, dentry slab was taking too much memory
and nearly all dentry there were negative.

The following is test result we run on two types of servers, one is 256G
memory with 24 CPUS and another is 3T memory with 384 CPUS. The test
case is using a lot of processes to generate negative dentry in
parallel, the following is the test result after 72 hours, the negative
dentry number is stable around that number even running longer time. If
without the patch set, in less than half an hour 197G was took by
negative dentry on 256G system, in 1 day 2.4T was took on 3T system.

neg-dentry-number        neg-dentry-mem-usage

256G 55259084 10.6G

3T 202306756 38.8G

For perf test, we run the following, and no regression found.

- create 1M negative dentry and then touch them to convert them to
positive dentry

- create 10K/100K/1M files

- remove 10K/100K/1M files

- kernel compile

To verify the fsnotify fix, we used inotifywait to watch file
create/open in some directory where there is a lot of negative dentry,
without the patch set, the system will run into soft lockup, with it, no
soft lockup.

We also try to defeat the limitation by making different processes
generating negative dentry with the same naming way, that will make one
negative dentry being accessed couple times around same time,
DCACHE_REFERENCED will be set on it and then it can't be trimmed easily.
We do see negative dentry will take all the memory slowly from one of
our system with 120G memory, for above two system, we see the memory
usage were increased, but still a small part of total memory. This looks
ok, since the common negative dentry user case will be create some temp
files and then remove it, it will be rare to access same negative dentry
around same time.

Thanks,

Junxiao.

On 5/8/20 5:23 AM, Konstantin Khlebnikov wrote:
> For most filesystems result of every negative lookup is cached, content of
> directories is usually cached too. Production of negative dentries isn't
> limited with disk speed. It's really easy to generate millions of them if
> system has enough memory.
>
> Getting this memory back ins't that easy because slab frees pages only when
> all related objects are gone. While dcache shrinker works in LRU order.
>
> Typical scenario is an idle system where some process periodically creates
> temporary files and removes them. After some time, memory will be filled
> with negative dentries for these random file names.
>
> Simple lookup of random names also generates negative dentries very fast.
> Constant flow of such negative denries drains all other inactive caches.
>
> Negative dentries are linked into siblings list along with normal positive
> dentries. Some operations walks dcache tree but looks only for positive
> dentries: most important is fsnotify/inotify. Hordes of negative dentries
> slow down these operations significantly.
>
> Time of dentry lookup is usually unaffected because hash table grows along
> with size of memory. Unless somebody especially crafts hash collisions.
>
> This patch set solves all of these problems:
>
> Move negative denries to the end of sliblings list, thus walkers could
> skip them at first sight (patches 3-6).
>
> Keep in dcache at most three unreferenced negative denties in row in each
> hash bucket (patches 7-8).
>
> ---
>
> Konstantin Khlebnikov (8):
> dcache: show count of hash buckets in sysctl fs.dentry-state
> selftests: add stress testing tool for dcache
> dcache: sweep cached negative dentries to the end of list of siblings
> fsnotify: stop walking child dentries if remaining tail is negative
> dcache: add action D_WALK_SKIP_SIBLINGS to d_walk()
> dcache: stop walking siblings if remaining dentries all negative
> dcache: push releasing dentry lock into sweep_negative
> dcache: prevent flooding with negative dentries
>
>
> fs/dcache.c | 144 +++++++++++-
> fs/libfs.c | 10 +-
> fs/notify/fsnotify.c | 6 +-
> include/linux/dcache.h | 6 +
> tools/testing/selftests/filesystems/Makefile | 1 +
> .../selftests/filesystems/dcache_stress.c | 210 ++++++++++++++++++
> 6 files changed, 370 insertions(+), 7 deletions(-)
> create mode 100644 tools/testing/selftests/filesystems/dcache_stress.c
>
> --
> Signature