Messages in this thread | | | Date | Tue, 29 Sep 2015 15:43:47 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 1/5] mm: uncharge kmem pages from generic free_page path |
| |
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.
| |