linux-kernel - Re: [PATCH 0/3] allow zram to use zbud as underlying allocator

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Date:	Thu, 17 Sep 2015 18:19:56 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Dan Streetman <ddstreet@...e.org>,
	Vitaly Wool <vitalywool@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 0/3] allow zram to use zbud as underlying allocator

On (09/17/15 08:21), Vlastimil Babka wrote:
> On 09/15/2015 06:22 AM, Sergey Senozhatsky wrote:
> >On (09/15/15 00:08), Dan Streetman wrote:
> >[..]
> >
> >correct. a bit of internals: we don't scan all the zspages every
> >time. each class has stats for allocated used objects, allocated
> >used objects, etc. so we 'compact' only classes that can be
> >compacted:
> >
> >  static unsigned long zs_can_compact(struct size_class *class)
> >  {
> >          unsigned long obj_wasted;
> >
> >          obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
> >                  zs_stat_get(class, OBJ_USED);
> >
> >          obj_wasted /= get_maxobj_per_zspage(class->size,
> >                          class->pages_per_zspage);
> >
> >          return obj_wasted * class->pages_per_zspage;
> >  }
> >
> >if we can free any zspages (which is at least one page), then we
> >attempt to do so.
> >
> >is compaction the root cause of the symptoms Vitaly observe?
> 
> He mentioned the "compact_stalls" counter which in /proc/vmstat is for the
> traditional physical memory compaction, not the zsmalloc-specific one. Which
> would imply high-order allocations. Does zsmalloc try them first before
> falling back to the order-0 zspages linked together manually?

each zspage is a bunch (pages_per_zspage) of alloc_page() calls

        for (i = 0; i < class->pages_per_zspage; i++) {
                struct page *page;

                page = alloc_page(flags);
                if (!page)
                        goto cleanup;

                INIT_LIST_HEAD(&page->lru);
                if (i == 0) {   /* first page */
                        SetPagePrivate(page);
                        set_page_private(page, 0);
                        first_page = page;
                        first_page->inuse = 0;
                }
                if (i == 1)
                        set_page_private(first_page, (unsigned long)page);
                if (i >= 1)
                        set_page_private(page, (unsigned long)first_page);
                if (i >= 2)
                        list_add(&page->lru, &prev_page->lru);
                if (i == class->pages_per_zspage - 1)   /* last page */
                        SetPagePrivate2(page);
                prev_page = page;
        }

	-ss
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