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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, May 18, 2020 at 07:04:40PM -0700, Yang Shi wrote:
> 
> 
> 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.

That's true. I missed that.

> > 
> > 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
> > > 
> > > 
> 

-- 
 Kirill A. Shutemov