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Date:   Thu, 14 Mar 2019 06:27:55 +0000
From:   Naoya Horiguchi <>
To:     zhong jiang <>
CC:     Minchan Kim <>, Michal Hocko <>,
        Vlastimil Babka <>,
        Linux Memory Management List <>,
        LKML <>
Subject: Re: [Qestion] Hit a WARN_ON_ONCE in try_to_unmap_one when runing


On Wed, Mar 13, 2019 at 12:03:20AM +0800, zhong jiang wrote:
> Minchan has changed the conditon check from  BUG_ON  to WARN_ON_ONCE in try_to_unmap_one.
> However,  It is still an abnormal condition when PageSwapBacked is not equal to PageSwapCache.
> But Is there any case it will meet the conditon in the mainline.
> It is assumed that PageSwapBacked(page) is true in the anonymous page,   This is to say,  PageSwapcache
> is false. however,  That is impossible because we will update the pte for hwpoison entry.
> Because page is locked ,  Its page flags should not be changed except for PageSwapBacked

try_to_unmap_one() from hwpoison_user_mappings() could reach the
WARN_ON_ONCE() only if TTU_IGNORE_HWPOISON is set, because PageHWPoison()
is set at the beginning of memory_failure().

Clearing TTU_IGNORE_HWPOISON might happen on the following two paths:

  static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
                                    int flags, struct page **hpagep)
      if (PageSwapCache(p)) {
              pr_err("Memory failure: %#lx: keeping poisoned page in swap cache\n",
              ttu |= TTU_IGNORE_HWPOISON;

      mapping = page_mapping(hpage);                                                                           
      if (!(flags & MF_MUST_KILL) && !PageDirty(hpage) && mapping &&                                           
          mapping_cap_writeback_dirty(mapping)) {                                                              
              if (page_mkclean(hpage)) {                                                                       
              } else {                                                                                         
                      kill = 0;                                                                                
                      ttu |= TTU_IGNORE_HWPOISON;                                                              
                      pr_info("Memory failure: %#lx: corrupted page was clean: dropped without side effects\n",

      unmap_success = try_to_unmap(hpage, ttu);

So either of the above "ttu |= TTU_IGNORE_HWPOISON" should be executed.
I'm not sure which one, but both paths show printk messages, so if you
could have kernel message log, that might help ...

Naoya Horiguchi

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