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.
>
> >Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >---
> > mm/debug.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> >diff --git a/mm/debug.c b/mm/debug.c
> >index 8865bfb41b0b..74c7cae4f683 100644
> >--- a/mm/debug.c
> >+++ b/mm/debug.c
> >@@ -42,9 +42,11 @@ const struct trace_print_flags vmaflag_names[] = {
> >
> > void __dump_page(struct page *page, const char *reason)
> > {
>
> At least there should be a comment explaining why.
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>
---
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",
--
Kirill A. Shutemov
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