On Tue, May 25, 2021 at 3:06 PM Hugh Dickins [off-list ref] wrote:

On Tue, 25 May 2021, Yang Shi wrote:

quoted

When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
return false positive for PTE-mapped THP since page_mapcount() is used
to check if the THP is unmapped, but it just checks compound mapount and
head page's mapcount.  If the THP is PTE-mapped and head page is not
mapped, it may return false positive.

But those false positives did not matter because there was a separate
DEBUG_VM check later.

It's good to have the link to Wang Yugui's report, but that paragraph
is not really about this patch, as it has evolved now: this patch
consolidates the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE.

quoted

The try_to_unmap() has been changed to void function, so check
page_mapped() after it.  And changed BUG_ON to WARN_ON since it is not a
fatal issue.

The change from DEBUG_VM BUG to VM_WARN_ON_ONCE is the most important
part of this, and the reason it's good for stable: and the patch title
ought to highlight that, not the page_mapcount business.

Will update the subject and the commit log accordingly.

quoted

[1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/ (local)

Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Yang Shi <redacted>

This will be required Cc: stable@vger.kernel.org
(but we don't want to Cc them on this mail).

As I said on the other, I think this should be 1/2 not 2/2.

Sure.

quoted

---
v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
    since there is no fundamental change against v2.
v2: Removed dead code and updated the comment of try_to_unmap() per Zi
    Yan.
 mm/huge_memory.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 80fe642d742d..72d81d8e01b1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -2343,6 +2343,8 @@ static void unmap_page(struct page *page)
              ttu_flags |= TTU_SPLIT_FREEZE;

      try_to_unmap(page, ttu_flags);
+
+     VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);

There is one useful piece of information that dump_page() will not show:
total_mapcount(page).  Is there a way of crafting that into the output?

Not with the macros available, I think.  Maybe we should be optimistic
and assume I already have the fixes, so not worth trying to refine the
message (but I'm not entirely convinced of that!).

The trouble with
        if (VM_WARN_ON_ONCE_PAGE(page_mapped(page), page))
                pr_warn("total_mapcount:%d\n", total_mapcount(page));
is that it's printed regardless of the ONCEness.  Another "trouble"
is that it's printed so long after the page_mapped(page) check that
it may be 0 by now - but one can see that as itself informative.

We should be able to make dump_page() print total mapcount, right? The
dump_page() should be just called in some error paths so taking some
extra overhead to dump more information seems harmless, or am I
missing something? Of course, this can be done in a separate patch.

I guess leave it as you have it, I don't see an easy better
(without going back to something like the old contortions).

quoted

 }

 static void remap_page(struct page *page, unsigned int nr)

@@ -2653,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
      struct deferred_split *ds_queue = get_deferred_split_queue(head);
      struct anon_vma *anon_vma = NULL;
      struct address_space *mapping = NULL;
-     int count, mapcount, extra_pins, ret;
+     int mapcount, extra_pins, ret;

Remove mapcount too.

quoted

      pgoff_t end;

      VM_BUG_ON_PAGE(is_huge_zero_page(head), head);

@@ -2712,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
      }

      unmap_page(head);
-     VM_BUG_ON_PAGE(compound_mapcount(head), head);

      /* block interrupt reentry in xa_lock and spinlock */
      local_irq_disable();

@@ -2730,7 +2731,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)

      /* Prevent deferred_split_scan() touching ->_refcount */
      spin_lock(&ds_queue->split_queue_lock);
-     count = page_count(head);
      mapcount = total_mapcount(head);
      if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {

mapcount was useful for printing in the hand-crafted message deleted,
but serves no purpose now: just do the page_ref_freeze() without it.

Aha, yes, good catch. If mapcount is not zero, the refcount freeze
won't succeed.

quoted

              if (!list_empty(page_deferred_list(head))) {

@@ -2752,16 +2752,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
              __split_huge_page(page, list, end);
              ret = 0;
      } else {
-             if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
-                     pr_alert("total_mapcount: %u, page_count(): %u\n",
-                                     mapcount, count);
-                     if (PageTail(page))
-                             dump_page(head, NULL);
-                     dump_page(page, "total_mapcount(head) > 0");
-                     BUG();
-             }
              spin_unlock(&ds_queue->split_queue_lock);
-fail:                if (mapping)
+fail:
+             if (mapping)
                      xa_unlock(&mapping->i_pages);
              local_irq_enable();
              remap_page(head, thp_nr_pages(head));
--

2.26.2