Messages in this thread | | | Subject | Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage() | From | Christophe Leroy <> | Date | Fri, 16 Apr 2021 13:08:25 +0200 |
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Le 16/04/2021 à 12:51, Steven Price a écrit : > On 16/04/2021 11:38, Christophe Leroy wrote: >> >> >> Le 16/04/2021 à 11:28, Steven Price a écrit : >>> On 15/04/2021 18:18, Christophe Leroy wrote: >>>> In order to support large pages on powerpc, notepage() >>>> needs to know the page size of the page. >>>> >>>> Add a page_size argument to notepage(). >>>> >>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu> >>>> --- >>>> arch/arm64/mm/ptdump.c | 2 +- >>>> arch/riscv/mm/ptdump.c | 2 +- >>>> arch/s390/mm/dump_pagetables.c | 3 ++- >>>> arch/x86/mm/dump_pagetables.c | 2 +- >>>> include/linux/ptdump.h | 2 +- >>>> mm/ptdump.c | 16 ++++++++-------- >>>> 6 files changed, 14 insertions(+), 13 deletions(-) >>>> >>> [...] >>>> diff --git a/mm/ptdump.c b/mm/ptdump.c >>>> index da751448d0e4..61cd16afb1c8 100644 >>>> --- a/mm/ptdump.c >>>> +++ b/mm/ptdump.c >>>> @@ -17,7 +17,7 @@ static inline int note_kasan_page_table(struct mm_walk *walk, >>>> { >>>> struct ptdump_state *st = walk->private; >>>> - st->note_page(st, addr, 4, pte_val(kasan_early_shadow_pte[0])); >>>> + st->note_page(st, addr, 4, pte_val(kasan_early_shadow_pte[0]), PAGE_SIZE); >>> >>> I'm not completely sure what the page_size is going to be used for, but note that KASAN presents >>> an interesting case here. We short-cut by detecting it's a KASAN region at a high level >>> (PGD/P4D/PUD/PMD) and instead of walking the tree down just call note_page() *once* but with >>> level==4 because we know KASAN sets up the page table like that. >>> >>> However the one call actually covers a much larger region - so while PAGE_SIZE matches the level >>> it doesn't match the region covered. AFAICT this will lead to odd results if you enable KASAN on >>> powerpc. >> >> Hum .... I successfully tested it with KASAN, I now realise that I tested it with >> CONFIG_KASAN_VMALLOC selected. In this situation, since >> https://github.com/torvalds/linux/commit/af3d0a686 we don't have any common shadow page table >> anymore. >> >> I'll test again without CONFIG_KASAN_VMALLOC. >> >>> >>> To be honest I don't fully understand why powerpc requires the page_size - it appears to be using >>> it purely to find "holes" in the calls to note_page(), but I haven't worked out why such holes >>> would occur. >> >> I was indeed introduced for KASAN. We have a first commit >> https://github.com/torvalds/linux/commit/cabe8138 which uses page size to detect whether it is a >> KASAN like stuff. >> >> Then came https://github.com/torvalds/linux/commit/b00ff6d8c as a fix. I can't remember what the >> problem was exactly, something around the use of hugepages for kernel memory, came as part of the >> series >> https://patchwork.ozlabs.org/project/linuxppc-dev/cover/cover.1589866984.git.christophe.leroy@csgroup.eu/ > > > Ah, that's useful context. So it looks like powerpc took a different route to reducing the KASAN > output to x86. > > Given the generic ptdump code has handling for KASAN already it should be possible to drop that from > the powerpc arch code, which I think means we don't actually need to provide page size to > notepage(). Hopefully that means more code to delete ;) >
Yes ... and no.
It looks like the generic ptdump handles the case when several pgdir entries points to the same kasan_early_shadow_pte. But it doesn't take into account the powerpc case where we have regular page tables where several (if not all) PTEs are pointing to the kasan_early_shadow_page .
Christophe
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