The patch titled
Subject: mm: thp: check total_mapcount instead of page_mapcount
has been added to the -mm tree. Its filename is
mm-thp-check-total_mapcount-instead-of-page_mapcount.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-thp-check-total_mapcount-instead-of-page_mapcount.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-check-total_mapcount-instead-of-page_mapcount.patch
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------------------------------------------------------
From: Yang Shi <shy828301@xxxxxxxxx>
Subject: mm: thp: check total_mapcount instead of page_mapcount
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 mapcount and
head page's mapcount. If the THP is PTE-mapped and head page is not
mapped, it may return a false positive.
Use total_mapcount() instead of page_mapcount() for try_to_unmap() and do
so for the VM_BUG_ON_PAGE in split_huge_page_to_list as well.
This changes the semantics of try_to_unmap(), but I don't see there is any
use case that expects try_to_unmap() to just unmap one subpage of a huge
page. So using page_mapcount() seems like a bug.
[1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@xxxxxxxxxxxx/
Link: https://lkml.kernel.org/r/20210513212334.217424-1-shy828301@xxxxxxxxx
Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Wang Yugui <wangyugui@xxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/huge_memory.c | 11 +----------
mm/rmap.c | 10 ++++++----
2 files changed, 7 insertions(+), 14 deletions(-)
--- a/mm/huge_memory.c~mm-thp-check-total_mapcount-instead-of-page_mapcount
+++ a/mm/huge_memory.c
@@ -2350,7 +2350,6 @@ static void unmap_page(struct page *page
ttu_flags |= TTU_SPLIT_FREEZE;
unmap_success = try_to_unmap(page, ttu_flags);
- VM_BUG_ON_PAGE(!unmap_success, page);
}
static void remap_page(struct page *page, unsigned int nr)
@@ -2720,7 +2719,7 @@ int split_huge_page_to_list(struct page
}
unmap_page(head);
- VM_BUG_ON_PAGE(compound_mapcount(head), head);
+ VM_BUG_ON_PAGE(total_mapcount(head), head);
/* block interrupt reentry in xa_lock and spinlock */
local_irq_disable();
@@ -2760,14 +2759,6 @@ int split_huge_page_to_list(struct page
__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)
xa_unlock(&mapping->i_pages);
--- a/mm/rmap.c~mm-thp-check-total_mapcount-instead-of-page_mapcount
+++ a/mm/rmap.c
@@ -1742,12 +1742,14 @@ static int page_not_mapped(struct page *
}
/**
- * try_to_unmap - try to remove all page table mappings to a page
- * @page: the page to get unmapped
+ * try_to_unmap - try to remove all page table mappings to a page and the
+ * compound page it belongs to
+ * @page: the page or the subpages of compound page to get unmapped
* @flags: action and flags
*
* Tries to remove all the page table entries which are mapping this
- * page, used in the pageout path. Caller must hold the page lock.
+ * page and the compound page it belongs to, used in the pageout path.
+ * Caller must hold the page lock.
*
* If unmap is successful, return true. Otherwise, false.
*/
@@ -1777,7 +1779,7 @@ bool try_to_unmap(struct page *page, enu
else
rmap_walk(page, &rwc);
- return !page_mapcount(page) ? true : false;
+ return !total_mapcount(page) ? true : false;
}
/**
_
Patches currently in -mm which might be from shy828301@xxxxxxxxx are
mm-thp-check-total_mapcount-instead-of-page_mapcount.patch
