On Fri, Jul 02, 2021 at 10:45:09AM +0800, Zhaoyang Huang wrote: > 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; That's the point here. With SLAB_RECLAIM_ACCOUNT, page allocator try to allocate pages from MIGRATE_RECLAIMABLE with belief those objects are easily reclaimable. Say a page has object A, B, C, D and E. A-D are easily reclaimable but E is hard. What happens is VM couldn't reclaim the page in the end due to E even though it already reclaimed A-D. And the such fragmenation could be spread out entire MIGRATE_RECLAIMABLE pageblocks over time. That's why I'd like to put zspage into MIGRATE_UNMOVALBE from the beginning since I don't think it's easily reclaimble once compaction is done.
