Re: [patch 108/192] mm: zram: amend SLAB_RECLAIM_ACCOUNT on zspage_cachep

[Zhaoyang Huang]

On Thu, Jul 1, 2021 at 10:56 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> On Wed, Jun 30, 2021 at 06:52:58PM -0700, Andrew Morton wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > Subject: mm: zram: amend SLAB_RECLAIM_ACCOUNT on zspage_cachep
> >
> > Zspage_cachep is found be merged with other kmem cache during test, which
> > is not good for debug things (zs_pool->zspage_cachep present to be another
> > kmem cache in memory dumpfile).  It is also neccessary to do so as
> > shrinker has been registered for zspage.
> >
> > Amending this flag can help kernel to calculate SLAB_RECLAIMBLE correctly.
> >
> > Link: https://lkml.kernel.org/r/1623137297-29685-1-git-send-email-huangzhaoyang@xxxxxxxxx
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>
> Sorry for the late. I don't think this is correct.
>
> It's true "struct zspage" can be freed by zsmalloc's compaction registerred
> by slab shrinker so tempted to make it SLAB_RECLAIM_ACCOUNT. However, it's
> quite limited to work only when objects in the zspage are heavily fragmented.
> Once the compaction is done, zspage are never discardable until objects are
> fragmented again. It means it could hurt other reclaimable slab page reclaiming
> since the zspage slab object pins the page.
IMHO, kmem cache's reclaiming is NOT affected by SLAB_RECLAIM_ACCOUNT
. This flag just affects kmem cache merge[1], the slab page's migrate
type[2] and the page's statistics. Actually, zspage's cache DO merged
with others even without SLAB_RECLAIM_ACCOUNT currently, which maybe
cause zspage's object will NEVER be discarded.(SLAB_MERGE_SAME
introduce confusions as people believe the cache will merge with
others when it set and vice versa)

[1]
 struct kmem_cache *find_mergeable(size_t size, size_t align, unsigned
long flags, const char *name, void (*ctor)(void *))
...
    if ((flags & SLAB_MERGE_SAME) != (s->flags & SLAB_MERGE_SAME))
     continue;

[2]
if (s->flags & SLAB_RECLAIM_ACCOUNT)
    s->allocflags |= __GFP_RECLAIMABLE;

>
> > ---
> >
> >  mm/zsmalloc.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > --- a/mm/zsmalloc.c~mm-zram-amend-slab_reclaim_account-on-zspage_cachep
> > +++ a/mm/zsmalloc.c
> > @@ -328,7 +328,7 @@ static int create_cache(struct zs_pool *
> >               return 1;
> >
> >       pool->zspage_cachep = kmem_cache_create("zspage", sizeof(struct zspage),
> > -                                     0, 0, NULL);
> > +                                     0, SLAB_RECLAIM_ACCOUNT, NULL);
> >       if (!pool->zspage_cachep) {
> >               kmem_cache_destroy(pool->handle_cachep);
> >               pool->handle_cachep = NULL;
> > _