Re: [RFC] mm: indirectly reclaimable memory and dcache

Andrew Morton
2018-03-01T22:55:49Z

On Thu, 1 Mar 2018 22:17:13 +0000 Roman Gushchin <guro@fb.com> wrote:

> I was reported about suspicious growth of unreclaimable slabs
> on some machines. I've found that it happens on machines
> with low memory pressure, and these unreclaimable slabs
> are external names attached to dentries.
>
> External names are allocated using generic kmalloc() function,
> so they are accounted as unreclaimable. But they are held
> by dentries, which are reclaimable, and they will be reclaimed
> under the memory pressure.
>
> In particular, this breaks MemAvailable calculation, as it
> doesn't take unreclaimable slabs into account.
> This leads to a silly situation, when a machine is almost idle,
> has no memory pressure and therefore has a big dentry cache.
> And the resulting MemAvailable is too low to start a new workload.
>
> To resolve this issue, a new mm counter is introduced:
> NR_INDIRECTLY_RECLAIMABLE_BYTES .
> Since it's not possible to count such objects on per-page basis,
> let's make the unit obvious (by analogy to NR_KERNEL_STACK_KB).
>
> The counter is increased in dentry allocation path, if an external
> name structure is allocated; and it's decreased in dentry freeing
> path. I believe, that it's not the only case in the kernel, when
> we do have such indirectly reclaimable memory, so I expect more
> use cases to be added.
>
> This counter is used to adjust MemAvailable calculations:
> indirectly reclaimable memory is considered as available.
>
> To reproduce the problem I've used the following Python script:
> import os
>
> for iter in range (0, 10000000):
> try:
> name = ("/some_long_name_%d" % iter) + "_" * 220
> os.stat(name)
> except Exception:
> pass
>
> Without this patch:
> $ cat /proc/meminfo | grep MemAvailable
> MemAvailable: 7811688 kB
> $ python indirect.py
> $ cat /proc/meminfo | grep MemAvailable
> MemAvailable: 2753052 kB
>
> With the patch:
> $ cat /proc/meminfo | grep MemAvailable
> MemAvailable: 7809516 kB
> $ python indirect.py
> $ cat /proc/meminfo | grep MemAvailable
> MemAvailable: 7749144 kB
>
> Also, this patch adds a corresponding entry to /proc/vmstat:
>
> $ cat /proc/vmstat | grep indirect
> nr_indirectly_reclaimable 5117499104
>
> $ echo 2 > /proc/sys/vm/drop_caches
>
> $ cat /proc/vmstat | grep indirect
> nr_indirectly_reclaimable 7104

hm, I guess so...

I wonder if it should be more general, as there are probably other
potential users of NR_INDIRECTLY_RECLAIMABLE_BYTES. And they might be
using alloc_pages() or even vmalloc()? Whereas
NR_INDIRECTLY_RECLAIMABLE_BYTES is pretty closely tied to kmalloc, at
least in the code comments.

If we're really OK with the "only for kmalloc" concept then why create
NR_INDIRECTLY_RECLAIMABLE_BYTES at all? Could we just use
NR_SLAB_RECLAIMABLE to account the external names? After all, kmalloc
is slab.