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. 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 changed the semantic of try_to_unmap(), but I don't see there is any usecase that expects try_to_unmap() 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/ Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> --- v2: Removed dead code and updated the comment of try_to_unmap() per Zi Yan. mm/huge_memory.c | 11 +---------- mm/rmap.c | 10 ++++++---- 2 files changed, 7 insertions(+), 14 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 63ed6b25deaa..3b08b9ba1578 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2348,7 +2348,6 @@ 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) @@ -2718,7 +2717,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) } 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(); @@ -2758,14 +2757,6 @@ 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) xa_unlock(&mapping->i_pages); diff --git a/mm/rmap.c b/mm/rmap.c index 693a610e181d..f52825b1330d 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1742,12 +1742,14 @@ static int page_not_mapped(struct page *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, enum ttu_flags flags) else rmap_walk(page, &rwc); - return !page_mapcount(page) ? true : false; + return !total_mapcount(page) ? true : false; } /** -- 2.26.2
