On Fri, Nov 25, 2016 at 09:42:07AM +0100, Vlastimil Babka wrote: > On 11/24/2016 03:23 PM, Dmitry Vyukov wrote: > > On Thu, Nov 24, 2016 at 2:49 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote: > >> On 11/18/2016 11:19 AM, Dmitry Vyukov wrote: > >>> > >>> Hello, > >>> > >>> I've got the following BUG while running syzkaller on > >>> a25f0944ba9b1d8a6813fd6f1a86f1bd59ac25a6 (4.9-rc5). Unfortunately it's > >>> not reproducible. > >>> > >>> kernel BUG at ./include/linux/mm.h:1743! > >>> invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN > >> > >> > >> Shouldn't there be also dump_page() output? Since you've hit this: > >> VM_BUG_ON_PAGE(page->pmd_huge_pte, page); > > > > Here it is: > > > > [ 250.326131] page:ffffea0000e196c0 count:1 mapcount:0 mapping: > > (null) index:0x0 > > [ 250.343393] flags: 0x1fffc0000000000() > > [ 250.345328] page dumped because: VM_BUG_ON_PAGE(page->pmd_huge_pte) > > [ 250.346780] ------------[ cut here ]------------ > > [ 250.347742] kernel BUG at ./include/linux/mm.h:1743! > > Yeah, as expected, not very useful for this particular BUG_ON :/ > > >> Anyway the output wouldn't contain the value of pmd_huge_pte or stuff that's > >> in union with it. I'd suggest adding a local patch that prints this in the > >> error case, in case the fuzzer hits it again. > >> > >> Heck, it might even make sense to print raw contents of struct page in > >> dump_page() as a catch-all solution? Should I send a patch? > > > > Yes, please send. > > We are moving towards continuous build without local patches. > > Something like this? > -------8<------- > From 2ac2c9b83d7c4c8be076c24246865a2ed01f9032 Mon Sep 17 00:00:00 2001 > From: Vlastimil Babka <vbabka@xxxxxxx> > Date: Fri, 25 Nov 2016 09:08:05 +0100 > Subject: [PATCH] mm, debug: print raw struct page data in __dump_page() > > The __dump_page() function is used when a page metadata inconsistency is > detected, either by standard runtime checks, or extra checks in CONFIG_DEBUG_VM > builds. It prints some of the relevant metadata, but not the whole struct page, > which is based on unions and interpretation is dependent on the context. > > This means that sometimes e.g. a VM_BUG_ON_PAGE() checks certain field, which > is however not printed by __dump_page() and the resulting bug report may then > lack clues that could help in determining the root cause. This patch solves > the problem by simply printing the whole struct page word by word, so no part > is missing, but the interpretation of the data is left to developers. This is > similar to e.g. x86_64 raw stack dumps. > > Example output: > > page:ffffea00000475c0 count:1 mapcount:0 mapping: (null) index:0x0 > flags: 0x100000000000400(reserved) > raw struct page data: > 0100000000000400 0000000000000000 0000000000000000 00000001ffffffff > ffffea00000475e0 ffffea00000475e0 0000000000000000 0000000000000000 > page dumped because: VM_BUG_ON_PAGE(1) > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > --- > mm/debug.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/mm/debug.c b/mm/debug.c > index 9feb699c5d25..9f67ad74d036 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -48,6 +48,8 @@ void __dump_page(struct page *page, const char *reason) > * encode own info. > */ > int mapcount = PageSlab(page) ? 0 : page_mapcount(page); > + int i; > + const int words_per_line = (sizeof(unsigned long) == 8) ? 4 : 8; > > pr_emerg("page:%p count:%d mapcount:%d mapping:%p index:%#lx", > page, page_ref_count(page), mapcount, > @@ -59,6 +61,21 @@ void __dump_page(struct page *page, const char *reason) > > pr_emerg("flags: %#lx(%pGp)\n", page->flags, &page->flags); > > + pr_alert("raw struct page data:"); Do we really need this line? I would like to keep dump_page() output as compact as possible. > + for (i = 0; i < sizeof(struct page) / sizeof(unsigned long); i++) { > + unsigned long *word_ptr; > + > + word_ptr = ((unsigned long *) page) + i; > + > + if ((i % words_per_line) == 0) { > + pr_cont("\n"); > + pr_alert(" %016lx", *word_ptr); > + } else { > + pr_cont(" %016lx", *word_ptr); 16 is a waste on 32-bit system. And it will produce too long lines. Maybe 'unsigned long long' a time? > + } > + } > + pr_cont("\n"); > + > if (reason) > pr_alert("page dumped because: %s\n", reason); > > -- > 2.10.2 > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a> -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>