Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 15 Mar 2018 07:36:57 +0000 | Subject | Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment" |
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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.
> ------------------------ > include/linux/mmzone.h: > pfn_valid(pfn) > valid_section(__nr_to_section(pfn_to_section_nr(pfn))) > return (section && (section->section_mem_map & SECTION_HAS_MEM_MAP)) > > arch/arm64/mm/init.c: > #ifdef CONFIG_HAVE_ARCH_PFN_VALID > int pfn_valid(unsigned long pfn) > { > return memblock_is_map_memory(pfn << PAGE_SHIFT); > } > EXPORT_SYMBOL(pfn_valid); > #endif > ------------------------ > > Also I already sent a fix to Andrew yesterday which was reported to > fix the loop. > > Moreover, you also reported this: > >> Early memory node ranges >> node 0: [mem 0x0000000080000000-0x00000000febeffff] >> node 0: [mem 0x00000000febf0000-0x00000000fefcffff] >> node 0: [mem 0x00000000fefd0000-0x00000000ff43ffff] >> node 0: [mem 0x00000000ff440000-0x00000000ff7affff] >> node 0: [mem 0x00000000ff7b0000-0x00000000ffffffff] >> node 0: [mem 0x0000000880000000-0x0000000fffffffff] >> Initmem setup node 0 [mem 0x0000000080000000-0x0000000fffffffff] >> pfn:febf0 oldnext:febf0 newnext:fe9ff >> pfn:febf0 oldnext:febf0 newnext:fe9ff >> pfn:febf0 oldnext:febf0 newnext:fe9ff >> etc etc > > I am wondering how come pfn_valid(0xfebf0) returns false here. Should > it be true or do I miss something? > > --nX
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