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From: Nhat Pham <nphamcs@gmail.com>
Date: Fri, 2 Feb 2024 16:39:52 -0800
Message-ID: <CAKEwX=PH5abBFCjYHL+d99v8MMwiASqP83aF2vSv1iwezX3UHA@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] selftests: add zswapin and no zswap tests
To: Yosry Ahmed <yosryahmed@google.com>
Cc: akpm@linux-foundation.org, riel@surriel.com, shuah@kernel.org, 
	hannes@cmpxchg.org, tj@kernel.org, lizefan.x@bytedance.com, 
	roman.gushchin@linux.dev, linux-mm@kvack.org, kernel-team@meta.com, 
	linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, 
	linux-kselftest@vger.kernel.org
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On Thu, Feb 1, 2024 at 1:08=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com> =
wrote:
>
> Hey Nhat,
>
> I have a few more comments, sorry for not catching everything the first
> time around.

No worries :)

>
> Adding Roman to CC.
>
> On Wed, Jan 31, 2024 at 07:27:18PM -0800, Nhat Pham wrote:
> > Add a selftest to cover the zswapin code path, allocating more memory
> > than the cgroup limit to trigger swapout/zswapout, then reading the
> > pages back in memory several times. This is inspired by a recently
> > encountered kernel crash on the zswapin path in our internal kernel,
> > which went undetected because of a lack of test coverage for this path.
> >
> > Add a selftest to verify that when memory.zswap.max =3D 0, no pages can=
 go
> > to the zswap pool for the cgroup.
> >
> > Suggested-by: Rik van Riel <riel@surriel.com>
> > Suggested-by: Yosry Ahmed <yosryahmed@google.com>
> > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > ---
> >  tools/testing/selftests/cgroup/test_zswap.c | 97 +++++++++++++++++++++
> >  1 file changed, 97 insertions(+)
> >
> > diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testin=
g/selftests/cgroup/test_zswap.c
> > index 32ce975b21d1..14d1f18f1098 100644
> > --- a/tools/testing/selftests/cgroup/test_zswap.c
> > +++ b/tools/testing/selftests/cgroup/test_zswap.c
> > @@ -60,6 +60,27 @@ static long get_zswpout(const char *cgroup)
> >       return cg_read_key_long(cgroup, "memory.stat", "zswpout ");
> >  }
> >
> > +static int allocate_bytes_and_read(const char *cgroup, void *arg)
>
> I think allocate_and_read_bytes() is easier to read, but I don't feel
> strongly about it.

Ah you're right. I'll fix that in the next version.

>
> > +{
> > +     size_t size =3D (size_t)arg;
> > +     char *mem =3D (char *)malloc(size);
> > +     int ret =3D 0;
> > +
> > +     if (!mem)
> > +             return -1;
> > +     for (int i =3D 0; i < size; i +=3D 4095)
> > +             mem[i] =3D 'a';
>
> cgroup_util.h defines PAGE_SIZE, see alloc_anon() for example.
>
> On that note, alloc_anon() is awfully close to allocate_bytes() below,
> perhaps we should consolidate them. The only difference I see is that
> alloc_anon() does not check for the allocation failure, but a lot of
> functions in cgroup_helpers.c don't, so it seems intentional for
> simplification.

Hmm I didn't know about this function. I think it was Domenico who
added allocate_bytes() for the initial zswap tests, and I've just been
piggybacking on it ever since:
https://github.com/torvalds/linux/commit/d9cfaf405b8ffe2c716b1ce4c82e0a19d5=
0951da

I can send a separate patch to clean this up later :) Doesn't seem that bad=
.

>
> > +
> > +     /* go through the allocated memory to (z)swap in and out pages */
> > +     for (int i =3D 0; i < size; i +=3D 4095) {
> > +             if (mem[i] !=3D 'a')
> > +                     ret =3D -1;
> > +     }
> > +
> > +     free(mem);
> > +     return ret;
> > +}
> > +
> >  static int allocate_bytes(const char *cgroup, void *arg)
> >  {
> >       size_t size =3D (size_t)arg;
> > @@ -133,6 +154,80 @@ static int test_zswap_usage(const char *root)
> >       return ret;
> >  }
> >
> > +/*
> > + * Check that when memory.zswap.max =3D 0, no pages can go to the zswa=
p pool for
> > + * the cgroup.
> > + */
> > +static int test_swapin_nozswap(const char *root)
> > +{
> > +     int ret =3D KSFT_FAIL;
> > +     char *test_group;
> > +     long zswpout;
> > +
> > +     /* Set up */
>
> I think this comment is unnecessary.

Fair!

>
> > +     test_group =3D cg_name(root, "no_zswap_test");
> > +
> > +     if (!test_group)
> > +             goto out;
> > +     if (cg_create(test_group))
> > +             goto out;
> > +     if (cg_write(test_group, "memory.max", "8M"))
> > +             goto out;
> > +     /* Disable zswap */
>
> I think this comment is unnecessary.

Fair!

>
> > +     if (cg_write(test_group, "memory.zswap.max", "0"))
> > +             goto out;
> > +
> > +     /* Allocate and read more than memory.max to trigger swapin */
> > +     if (cg_run(test_group, allocate_bytes_and_read, (void *)MB(32)))
> > +             goto out;
> > +
> > +     /* Verify that no zswap happened */
>
> If we want to be really meticulous, we can verify that we did swap out,
> but not to zswap. IOW, we can check memory.swap.current or something.

Hmm would memory.swap.current go back to 0 once the memory-in-swap is
freed? It doesn't seem like we have any counters at the cgroup level
for swapout/swapin events. Maybe such counters were not useful enough
to justify the extra overhead of maintaining them? :)

Anyway, I think checking zswpout should probably be enough here.
That's the spirit of the test anyway - make absolutely sure that no
zswap-out happened.

>
> > +     zswpout =3D get_zswpout(test_group);
> > +     if (zswpout < 0) {
> > +             ksft_print_msg("Failed to get zswpout\n");
> > +             goto out;
> > +     } else if (zswpout > 0) {
>
> nit: This can be a separate if condition, I think it would be more
> inline with the style of separate consecutive if blocks we are
> following.

Yeah now that you point out the inconsistency, it starts to bug my
brain too :) I'll fix it in the next version.

>
> > +             ksft_print_msg(
> > +                     "Pages should not go to zswap when memory.zswap.m=
ax =3D 0\n");
>
> We can probably avoid the line break with something more concise, for
> example:
> "zswapout > 0 when zswap is disabled"
> or "zswapout > 0 when memory.zswap.max =3D 0"
>
> > +             goto out;
> > +     }
> > +     ret =3D KSFT_PASS;
> > +
> > +out:
> > +     cg_destroy(test_group);
> > +     free(test_group);
> > +     return ret;
> > +}
> > +
> > +/* Simple test to verify the (z)swapin code paths */
> > +static int test_zswapin_no_limit(const char *root)
>
> I think test_zswapin() is enough to be distinct from
> test_swapin_nozswap(). The limit is not a factor here AFAICT.

Fair.

>
> > +{
> > +     int ret =3D KSFT_FAIL;
> > +     char *test_group;
> > +
> > +     /* Set up */
>
> I think this comment is unnecessary.

Fair.

>
> > +     test_group =3D cg_name(root, "zswapin_test");
> > +     if (!test_group)
> > +             goto out;
> > +     if (cg_create(test_group))
> > +             goto out;
> > +     if (cg_write(test_group, "memory.max", "8M"))
> > +             goto out;
> > +     if (cg_write(test_group, "memory.zswap.max", "max"))
> > +             goto out;
> > +
> > +     /* Allocate and read more than memory.max to trigger (z)swap in *=
/
> > +     if (cg_run(test_group, allocate_bytes_and_read, (void *)MB(32)))
> > +             goto out;
>
> We should probably check for a positive zswapin here, no?

Oh right. I'll just do a quick check here:

zswpin =3D cg_read_key_long(test_group, "memory.stat", "zswpin ");
if (zswpin < 0) {
   ksft_print_msg("Failed to get zswpin\n");
   goto out;
}

if (zswpin =3D=3D 0) {
   ksft_print_msg("zswpin should not be 0\n");
   goto out;
}


>
> > +
> > +     ret =3D KSFT_PASS;
> > +
> > +out:
> > +     cg_destroy(test_group);
> > +     free(test_group);
> > +     return ret;
> > +}
> > +
> >  /*
> >   * When trying to store a memcg page in zswap, if the memcg hits its m=
emory
> >   * limit in zswap, writeback should affect only the zswapped pages of =
that
> > @@ -309,6 +404,8 @@ struct zswap_test {
> >       const char *name;
> >  } tests[] =3D {
> >       T(test_zswap_usage),
> > +     T(test_swapin_nozswap),
> > +     T(test_zswapin_no_limit),
> >       T(test_no_kmem_bypass),
> >       T(test_no_invasive_cgroup_shrink),
> >  };
> > --
> > 2.39.3