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    SubjectRe: [External] Re: [PATCH] mm/memcontrol: make the slab calculation consistent
    On Thu, Dec 3, 2020 at 11:21 AM Roman Gushchin <guro@fb.com> wrote:
    >
    > On Thu, Dec 03, 2020 at 10:53:33AM +0800, Muchun Song wrote:
    > > On Thu, Dec 3, 2020 at 5:16 AM Roman Gushchin <guro@fb.com> wrote:
    > > >
    > > > On Wed, Dec 02, 2020 at 08:14:34PM +0800, Muchun Song wrote:
    > > > > Although the ratio of the slab is one, we also should read the ratio
    > > > > from the related memory_stats instead of hard-coding. And the local
    > > > > variable of size is already the value of slab_unreclaimable. So we
    > > > > do not need to read again. Simplify the code here.
    > > > >
    > > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
    > > > > ---
    > > > > mm/memcontrol.c | 22 +++++++++++++++++-----
    > > > > 1 file changed, 17 insertions(+), 5 deletions(-)
    > > >
    > > > Hi Muchun!
    > > >
    > > > >
    > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > > > > index 9922f1510956..03a9c64560f6 100644
    > > > > --- a/mm/memcontrol.c
    > > > > +++ b/mm/memcontrol.c
    > > > > @@ -1545,12 +1545,22 @@ static int __init memory_stats_init(void)
    > > > > int i;
    > > > >
    > > > > for (i = 0; i < ARRAY_SIZE(memory_stats); i++) {
    > > > > + switch (memory_stats[i].idx) {
    > > > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > > > > - if (memory_stats[i].idx == NR_ANON_THPS ||
    > > > > - memory_stats[i].idx == NR_FILE_THPS ||
    > > > > - memory_stats[i].idx == NR_SHMEM_THPS)
    > > > > + case NR_ANON_THPS:
    > > > > + case NR_FILE_THPS:
    > > > > + case NR_SHMEM_THPS:
    > > > > memory_stats[i].ratio = HPAGE_PMD_SIZE;
    > > > > + break;
    > > > > #endif
    > > > > + case NR_SLAB_UNRECLAIMABLE_B:
    > > > > + VM_BUG_ON(i < 1);
    > > > > + VM_BUG_ON(memory_stats[i - 1].idx != NR_SLAB_RECLAIMABLE_B);
    > > >
    > > > Please, convert these to BUILD_BUG_ON(), they don't have to be runtime checks.
    > >
    > > Agree. But here we cannot use BUILD_BUG_ON(). The compiler will
    > > complain about it.
    >
    > We can!
    >
    > We just need to change the condition. All we really need to check is that
    > NR_SLAB_UNRECLAIMABLE_B immediately following NR_SLAB_RECLAIMABLE_B.

    But I think that we need to check that memory_stats[i] immediately following
    memory_stats[j] where i is the index of NR_SLAB_UNRECLAIMABLE_B and
    j is the index of NR_SLAB_RECLAIMABLE_B.

    >
    > Something like BUILD_BUG_ON(NR_SLAB_UNRECLAIMABLE_B != NR_SLAB_RECLAIMABLE_B + 1)

    So this cannot work. Thanks.

    > should work (completely untested).

    >
    > >
    > > >
    > > >
    > > > > + break;
    > > > > + default:
    > > > > + break;
    > > > > + }
    > > > > +
    > > > > VM_BUG_ON(!memory_stats[i].ratio);
    > > > > VM_BUG_ON(memory_stats[i].idx >= MEMCG_NR_STAT);
    > > > > }
    > > > > @@ -1587,8 +1597,10 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
    > > > > seq_buf_printf(&s, "%s %llu\n", memory_stats[i].name, size);
    > > > >
    > > >
    > > > Can you, please, add a small comment here stating that we're printing
    > > > unreclaimable, reclaimable and the sum of both? It will simplify the reading of the code.
    > >
    > > Will do.
    >
    > Thank you!



    --
    Yours,
    Muchun

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