Re: [PATCH 1/5] mm: uncharge kmem pages from generic free_page path

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 26 Sep 2015 13:45:53 +0300 Vladimir Davydov <vdavydov@parallels.com> wrote:

> Currently, to charge a page to kmemcg one should use alloc_kmem_pages
> helper. When the page is not needed anymore it must be freed with
> free_kmem_pages helper, which will uncharge the page before freeing it.
> Such a design is acceptable for thread info pages and kmalloc large
> allocations, which are currently the only users of alloc_kmem_pages, but
> it gets extremely inconvenient if one wants to make use of batched free
> (e.g. to charge page tables - see release_pages) or page reference
> counter (pipe buffers - see anon_pipe_buf_release).
> 
> To overcome this limitation, this patch moves kmemcg uncharge code to
> the generic free path and zaps free_kmem_pages helper. To distinguish
> kmem pages from other page types, it makes alloc_kmem_pages initialize
> page->_mapcount to a special value and introduces a new PageKmem helper,
> which returns true if it sees this value.

As far as I can tell, this new use of page._mapcount is OK, but...

- The documentation for _mapcount needs to be updated (mm_types.h)

- Don't believe the documentation!  Because someone else may have
  done what you tried to do.  Please manually audit mm/ for _mapcount
  uses.

- One such use is "For recording whether a page is in the buddy
  system, we set ->_mapcount PAGE_BUDDY_MAPCOUNT_VALUE".  Please update
  the comment for this while you're in there.  (Including description
  of the state's lifetime).

- And please update _mapcount docs for PageBalloon()

- Why is the code accessing ->_mapcount directly?  afaict page_mapcount()
  and friends will work OK?

- The patch adds overhead to all kernels, even non-kmemcg and
  non-memcg kernels.  Bad.  Fixable?

- PAGE_BUDDY_MAPCOUNT_VALUE, PAGE_BALLOON_MAPCOUNT_VALUE and
  PAGE_KMEM_MAPCOUNT_VALUE should all be put next to each other so
  readers can see all the possible values and so we don't get
  duplicates, etc.


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