Messages in this thread | | | Subject | Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage() | From | Steven Price <> | Date | Fri, 16 Apr 2021 14:00:28 +0100 |
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On 16/04/2021 12:08, Christophe Leroy wrote: > > > 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 .
I'm not sure I follow quite how powerpc is different here. But could you have a similar check for PTEs against kasan_early_shadow_pte as the other levels already have?
I'm just worried that page_size isn't well defined in this interface and it's going to cause problems in the future.
Steve
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