Re: [RFC linux-next PATCH] mm: khugepaged: remove error message when checking external pins

[Yang Shi <yang.shi@linux.alibaba.com>]

On 5/18/20 3:19 AM, Kirill A. Shutemov wrote:
> On Wed, May 13, 2020 at 05:03:03AM +0800, Yang Shi wrote:
>> When running khugepaged with higher frequency (for example, set
>> scan_sleep_millisecs to 0), the below error message was reported:
>>
>> khugepaged: expected_refcount (1024) > refcount (512)
>> page:ffffd75784258000 count:511 mapcount:1 mapping:ffff968de06c7421 index:0x7fa288600
>>   compound_mapcount: 0
>>   flags: 0x17fffc00009003c(uptodate|dirty|lru|active|head|swapbacked)
>>   raw: 017fffc00009003c ffffd7578ba70788 ffffd7578bdb5148 ffff968de06c7421
>>   raw: 00000007fa288600 0000000000000000 000001ff00000000 ffff968e5e7d6000
>>   page dumped because: Unexpected refcount
>>   page->mem_cgroup:ffff968e5e7d6000
>>
>> This is introduced by allowing collapsing fork shared and PTE-mapped
>> THPs.  The check may run into the below race:
>>
>> Assuming parent process forked child process, then they do
>>
>> 	CPU A		CPU B			CPU C
>> 	-----		-----			-----
>> Parent			Child			khugepaged
>>
>> MADV_DONTNEED
>>    split huge pmd
>>    Double mapped
>> 			MADV_DONTNEED
>> 			  zap_huge_pmd
>> 			    remove_page_rmap
>> 			      Clear double map
>> 						khugepaged_scan_pmd(parent)
>> 						  check mapcount and refcount
>> 						  --> total_mapcount > refcount
>> 			      dec mapcount
>>
>> The issue can be reproduced by the below test program.
> Good catch! Thanks. And the fix looks reasnable.
>
> We might want to have a similar debug check in near !is_refcount_suitable()
> case in __collapse_huge_page_isolate(). The function is called with
> anon_vma lock taken on write and it should prevent the false-positive.

However it seems MADV_DONTNEED path doesn't take anon_vma lock.

>
> Anyway:
>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Thanks.

>
>> ---8<---
>> void main()
>> {
>>          void *addr;
>>          int ret;
>>          pid_t pid;
>>
>>          addr = memalign(ALIGN, 2 * 1024 * 1024);
>>          if (!addr) {
>>                  printf("malloc failed\n");
>>                  return;
>>          }
>>
>>          ret = madvise(addr, 2 * 1024 * 1024, MADV_HUGEPAGE);
>>          if (ret < 0) {
>>                  printf("madvise failed\n");
>>                  return;
>>          }
>>
>>          memset(addr, 0xdeadbeef, 2 * 1024 * 1024);
>>
>>          pid = fork();
>>
>>          if (pid == 0) {
>>                  /* Child process */
>>                  ret = madvise(addr + (2 * 1024 * 1024) - 4096, 4096, MADV_DONTNEED);
>>                  if (ret < 0) {
>>                          printf("madvise failed in child\n");
>>                          return;
>>                  }
>>                  sleep(120);
>>          } else if (pid > 0) {
>>                  sleep(5);
>>                  /* Parent process */
>>                  ret = madvise(addr, 2 * 1024 * 1024, MADV_DONTNEED);
>>                  if (ret < 0) {
>>                          printf("madvise failed in parent\n");
>>                          return;
>>                  }
>>          } else {
>>                  printf("fork failed\n");
>>                  return;
>>          }
>>
>>          sleep(120);
>> }
>> ---8<---
>>
>> So, total_mapcount > refcount seems not unexpected due to the inherent
>> race.  Removed the error message even though it is protected by
>> CONFIG_VM_DEBUG since we have to live with the race and AFAIK some
>> distros may have CONFIG_VM_DEBUG enabled dy default.
>>
>> Since such case is ephemeral we could always try collapse the area again
>> later, so it sounds not harmful.  But, it might report false positive if
>> the page has excessive GUP pins (i.e. 512), however it might be not that
>> bad since the same check will be done later.  I didn't figure out a
>> simple way to prevent the false positive.
>>
>> Added some notes to elaborate the race and the consequence as well.
>>
>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> ---
>>   mm/khugepaged.c | 24 +++++++++++++++++-------
>>   1 file changed, 17 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 1fdd677..048f5d4 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -602,12 +602,6 @@ static bool is_refcount_suitable(struct page *page)
>>   	if (PageSwapCache(page))
>>   		expected_refcount += compound_nr(page);
>>   
>> -	if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
>> -		pr_err("expected_refcount (%d) > refcount (%d)\n",
>> -				expected_refcount, refcount);
>> -		dump_page(page, "Unexpected refcount");
>> -	}
>> -
>>   	return page_count(page) == expected_refcount;
>>   }
>>   
>> @@ -1341,7 +1335,23 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>>   			goto out_unmap;
>>   		}
>>   
>> -		/* Check if the page has any GUP (or other external) pins */
>> +		/*
>> +		 * Check if the page has any GUP (or other external) pins.
>> +		 *
>> +		 * Here the check is racy it may see totmal_mapcount > refcount
>> +		 * in some cases.
>> +		 * For example, one process with one forked child process.
>> +		 * The parent has the PMD split due to MADV_DONTNEED, then
>> +		 * the child is trying unmap the whole PMD, but khugepaged
>> +		 * may be scanning the parent between the child has
>> +		 * PageDoubleMap flag cleared and dec the mapcount.  So
>> +		 * khugepaged may see total_mapcount > refcount.
>> +		 *
>> +		 * But such case is ephemeral we could always retry collapse
>> +		 * later.  However it may report false positive if the page
>> +		 * has excessive GUP pins (i.e. 512).  Anyway the same check
>> +		 * will be done again later the risk seems low.
>> +		 */
>>   		if (!is_refcount_suitable(page)) {
>>   			result = SCAN_PAGE_COUNT;
>>   			goto out_unmap;
>> -- 
>> 1.8.3.1
>>
>>