Re: [PATCH v3 0/5] HWpoison: further fixes and cleanups

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Tue, Sep 15, 2020 at 05:22:22PM -0400, Aristeu Rozanski wrote:
> Hi Oscar, Naoya,
> 
> On Mon, Sep 14, 2020 at 12:15:54PM +0200, Oscar Salvador wrote:
> > The important bit of this patchset is patch#1, which is a fix to take off
> > HWPoison pages off a buddy freelist since it can lead us to having HWPoison
> > pages back in the game without no one noticing it.
> > So fix it (we did that already for soft_offline_page [1]).
> > 
> > The other patches are clean-ups and not that important, so if anything,
> > consider patch#1 for inclusion.
> > 
> > [1] https://patchwork.kernel.org/cover/11704083/
> 
> I found something strange with your and Naoya's hwpoison rework. We have a
> customer with a testcase that basically does:
> 
> 	p1 = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> 	p2 = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> 
> 	madvise(p1, size, MADV_MERGEABLE);
> 	madvise(p2, size, MADV_MERGEABLE);
> 
> 	memset(p1, 'a', size);
> 	memset(p2, 'a', size);
> 
> 	madvise(p1, size, MADV_SOFT_OFFLINE);
> 
> 	madvise(p1, size, MADV_UNMERGEABLE);
> 	madvise(p2, size, MADV_UNMERGEABLE);
> 
> 
> where size is about 200,000 pages. It works on a x86_64 box (with and without the
> hwpoison rework). On ppc64 boxes (tested 3 different ones with at least 250GB memory)
> it fails to take a page off the buddy list (page_handle_poison()/take_page_off_buddy())
> (madvise MADV_SOFT_OFFLINE returns -EBUSY). Without the hwpoison rework the test passes.

I reproduced the similar -EBUSY with small average x86 VM, where it seems to me
a race between page_take_off_buddy() and page allocation.  Oscar's debug patch
shows the following kernel messages:

    [  627.357009] Soft offlining pfn 0x235018 at process virtual address 0x7fd112140000
    [  627.358747] __get_any_page: 0x235018 free buddy page
    [  627.359875] page:00000000038b52c9 refcount:0 mapcount:-128 mapping:0000000000000000 index:0x1 pfn:0x235018
    [  627.362002] flags: 0x57ffe000000000()
    [  627.362841] raw: 0057ffe000000000 fffff84648d12688 ffff955abffd1dd0 0000000000000000
    [  627.364555] raw: 0000000000000001 0000000000000000 00000000ffffff7f 0000000000000000
    [  627.366258] page dumped because: page_handle_poison
    [  627.367357] page->mem_cgroup:ffff9559b6912000
    [  627.368342] page_handle_poison: hugepage_or_freepage failed\xb8n
    [  627.368344] soft_offline_free_page: page_handle_poison -EBUSY
    [  627.370901] page:00000000038b52c9 refcount:6 mapcount:3 mapping:000000001226bf89 index:0x2710 pfn:0x235018
    [  627.373048] aops:ext4_da_aops ino:c63f3 dentry name:"system.journal"
    [  627.374526] flags: 0x57ffe00000201c(uptodate|dirty|lru|private)
    [  627.375865] raw: 0057ffe00000201c fffff84648d300c8 ffff955ab8c3f020 ffff955aba5f4ee0
    [  627.377586] raw: 0000000000002710 ffff9559b811fc98 0000000500000002 ffff9559b6912000
    [  627.379308] page dumped because: soft_offline_free_page
    [  627.380480] page->mem_cgroup:ffff9559b6912000

    CPU 0                                CPU 1

    get_any_page // returns 0 (free buddy path)
      soft_offline_free_page
                                         the page is allocated
        page_handle_poison -> fail
          return -EBUSY

I'm still not sure why this issue is invisible before rework patch,
but setting migrate type to MIGRATE_ISOLATE during offlining could affect
the behavior sensitively.

Thanks,
Naoya Horiguchi