Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 15 Mar 2018 07:45:21 +0000 | Subject | Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
| |
On 15 March 2018 at 07:44, Daniel Vacek <neelx@redhat.com> wrote: > On Thu, Mar 15, 2018 at 8:36 AM, Ard Biesheuvel > <ard.biesheuvel@linaro.org> wrote: >> On 15 March 2018 at 02:23, Daniel Vacek <neelx@redhat.com> wrote: >>> On Wed, Mar 14, 2018 at 8:29 PM, Ard Biesheuvel >>> <ard.biesheuvel@linaro.org> wrote: >>>> This reverts commit 864b75f9d6b0100bb24fdd9a20d156e7cda9b5ae. >>>> >>>> Commit 864b75f9d6b0 ("mm/page_alloc: fix memmap_init_zone pageblock >>>> alignment") modified the logic in memmap_init_zone() to initialize >>>> struct pages associated with invalid PFNs, to appease a VM_BUG_ON() >>>> in move_freepages(), which is redundant by its own admission, and >>>> dereferences struct page fields to obtain the zone without checking >>>> whether the struct pages in question are valid to begin with. >>>> >>>> Commit 864b75f9d6b0 only makes it worse, since the rounding it does >>>> may cause pfn assume the same value it had in a prior iteration of >>>> the loop, resulting in an infinite loop and a hang very early in the >>>> boot. Also, since it doesn't perform the same rounding on start_pfn >>>> itself but only on intermediate values following an invalid PFN, we >>>> may still hit the same VM_BUG_ON() as before. >>>> >>>> So instead, let's fix this at the core, and ensure that the BUG >>>> check doesn't dereference struct page fields of invalid pages. >>>> >>>> Fixes: 864b75f9d6b0 ("mm/page_alloc: fix memmap_init_zone pageblock alignment") >>>> Cc: Daniel Vacek <neelx@redhat.com> >>>> Cc: Mel Gorman <mgorman@techsingularity.net> >>>> Cc: Michal Hocko <mhocko@suse.com> >>>> Cc: Paul Burton <paul.burton@imgtec.com> >>>> Cc: Pavel Tatashin <pasha.tatashin@oracle.com> >>>> Cc: Vlastimil Babka <vbabka@suse.cz> >>>> Cc: Andrew Morton <akpm@linux-foundation.org> >>>> Cc: Linus Torvalds <torvalds@linux-foundation.org> >>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> >>>> --- >>>> mm/page_alloc.c | 13 +++++-------- >>>> 1 file changed, 5 insertions(+), 8 deletions(-) >>>> >>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>> index 3d974cb2a1a1..635d7dd29d7f 100644 >>>> --- a/mm/page_alloc.c >>>> +++ b/mm/page_alloc.c >>>> @@ -1910,7 +1910,9 @@ static int move_freepages(struct zone *zone, >>>> * Remove at a later date when no bug reports exist related to >>>> * grouping pages by mobility >>>> */ >>>> - VM_BUG_ON(page_zone(start_page) != page_zone(end_page)); >>>> + VM_BUG_ON(pfn_valid(page_to_pfn(start_page)) && >>>> + pfn_valid(page_to_pfn(end_page)) && >>>> + page_zone(start_page) != page_zone(end_page)); >>> >>> Hi, I am on vacation this week and I didn't have a chance to test this >>> yet but I am not sure this is correct. Generic pfn_valid() unlike the >>> arm{,64} arch specific versions returns true for all pfns in a section >>> if there is at least some memory mapped in that section. So I doubt >>> this prevents the crash I was targeting. I believe pfn_valid() does >>> not change a thing here :( >>> >> >> If this is the case, memblock_next_valid_pfn() is broken since it >> skips valid PFNs, and we should be fixing that instead. > > How do you define valid pfn? Maybe the generic version of pfn_valid() > should be fixed??? >
memblock_next_valid_pfn() skips PFNs for which pfn_valid() returns true. That is clearly a bug.
| |