Re: [PATCH 1/3] mm: Print head flags in dump_page

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 29, 2020 at 03:38:13PM -0700, John Hubbard wrote:
> On 2020-06-29 08:19, Matthew Wilcox (Oracle) wrote:
> > Tail page flags contain very little useful information.  Print the head
> > page's flags instead (even though PageHead is a little misleading).
> 
> You are right about the tail page. And the raw output provides the tail
> page flags, in case someone *really* needs to dig into tail page problems,
> so that's all good.
> 
> However, I just gave this a spin, and seeing the "|head" in the list for
> my "tail page: dump_page test" is also slightly misleading for me, too.

We could also do ...

@@ -48,6 +48,8 @@ void __dump_page(struct page *page, const char *reason)
        struct address_space *mapping;
        bool page_poisoned = PagePoisoned(page);
        bool compound = PageCompound(page);
+       unsigned long flags = page->flags;
+
        /*
         * Accessing the pageblock without the zone lock. It could change to
         * "isolate" again in the meantime, but since we are just dumping the
@@ -165,7 +162,9 @@ void __dump_page(struct page *page, const char *reason)
 out_mapping:
        BUILD_BUG_ON(ARRAY_SIZE(pageflag_names) != __NR_PAGEFLAGS + 1);
 
-       pr_warn("%sflags: %#lx(%pGp)%s\n", type, head->flags, &head->flags,
+       if (head != page)
+               flags = head->flags & ~PG_head;
+       pr_warn("%sflags: %#lx(%pGp)%s\n", type, flags, &flags,
                page_cma ? " CMA" : "");
 
 hex_only: