Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 15 Mar 2018 15:17:19 +0000 | Subject | Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
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On 15 March 2018 at 15:12, Daniel Vacek <neelx@redhat.com> wrote: > On Thu, Mar 15, 2018 at 8:45 AM, Ard Biesheuvel > <ard.biesheuvel@linaro.org> wrote: >> 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. > > So pfn_valid() does not mean this frame is usable memory? >
Who cares what it *means*?
memblock_next_valid_pfn() has 'valid_pfn' in its name, so if passing pfn A returns B, and there exists a C such that A < C < B and pfn_valid(C) returns true, memblock_next_valid_pfn doesn't do what it says on the tin and should be fixed.
You keep going on about how pfn_valid() does or does not do what you think, but that is really irrelevant.
> OK, in that case we need to fix or revert memblock_next_valid_pfn(). > That works for me. >
OK. You can add my ack to a patch that reverts it, and we can revisit it for the next cycle.
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