On 09/22/2016 12:55 PM, Kirill A. Shutemov wrote:
On Wed, Sep 21, 2016 at 04:27:31PM +0200, Vlastimil Babka wrote:
On 09/08/2016 10:21 AM, Kirill A. Shutemov wrote:
>dump_page() uses page_mapcount() to get mapcount of the page.
>page_mapcount() has VM_BUG_ON_PAGE(PageSlab(page)) as mapcount doesn't
>make sense for slab pages and the field in struct page used for other
>information.
>
>It leads to recursion if dump_page() called for slub page and DEBUG_VM
>is enabled:
>
>dump_page() -> page_mapcount() -> VM_BUG_ON_PAGE() -> dump_page -> ...
>
>Let's avoid calling page_mapcount() for slab pages in dump_page().
How about instead splitting page_mapcount() so that there is a version
without VM_BUG_ON_PAGE()?
Why? page->_mapping is garbage for slab page and might be confusing.
If you want the information from page->_mapping union for slab page to be
shown during dump_page() we should present in proper way.
Hmm, fair enough.
>+ int mapcount = PageSlab(page) ? 0 : page_mapcount(page);
From d550530cc40ca2e9d60c84a893901c2dad6e7767 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Date: Thu, 22 Sep 2016 13:52:40 +0300
Subject: [PATCH] mm: clarify why we avoid page_mapcount() for slab pages in
dump_page()
Let's add comment on why we skip page_mapcount() for sl[aou]b pages.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Thanks.
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
mm/debug.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/mm/debug.c b/mm/debug.c
index 74c7cae4f683..9feb699c5d25 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -42,6 +42,11 @@ const struct trace_print_flags vmaflag_names[] = {
void __dump_page(struct page *page, const char *reason)
{
+ /*
+ * Avoid VM_BUG_ON() in page_mapcount().
+ * page->_mapcount space in struct page is used by sl[aou]b pages to
+ * encode own info.
+ */
int mapcount = PageSlab(page) ? 0 : page_mapcount(page);
pr_emerg("page:%p count:%d mapcount:%d mapping:%p index:%#lx",
--
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