Messages in this thread | | | From | Daniel Vacek <> | Date | Thu, 15 Mar 2018 16:12:20 +0100 | Subject | Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
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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?
OK, in that case we need to fix or revert memblock_next_valid_pfn(). That works for me.
--nX
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