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 17:15:23 +0200 |
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Le 16/04/2021 à 17:04, Christophe Leroy a écrit : > > > Le 16/04/2021 à 16:40, Christophe Leroy a écrit : >> >> >> Le 16/04/2021 à 15:00, Steven Price a écrit : >>> 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 : >>>>>>> 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. >>> >> >> I'm trying. I reverted the two commits b00ff6d8c and cabe8138. >> >> At the moment, I don't get exactly what I expect: For linear memory I get one line for each 8M >> page whereas before reverting the patches I got one 16M line and one 112M line. >> >> And for KASAN shadow area I get two lines for the 2x 8M pages shadowing linear mem then I get one >> 4M line for each PGDIR entry pointing to kasan_early_shadow_pte. >> >> 0xf8000000-0xf87fffff 0x07000000 8M huge rw present >> 0xf8800000-0xf8ffffff 0x07800000 8M huge rw present >> 0xf9000000-0xf93fffff 0x01430000 4M r present > ... >> 0xfec00000-0xfeffffff 0x01430000 4M r present >> >> Any idea ? >> > > > I think the different with other architectures is here: > > } else if (flag != st->current_flags || level != st->level || > addr >= st->marker[1].start_address || > pa != st->last_pa + PAGE_SIZE) { > > > In addition to the checks everyone do, powerpc also checks "pa != st->last_pa + PAGE_SIZE". > And it is definitely for that test that page_size argument add been added.
By replacing that test by (pa - st->start_pa != addr - st->start_address) it works again. So we definitely don't need the real page size.
> > I see that other architectures except RISCV don't dump the physical address. But even RISCV doesn't > include that check. > > That physical address dump was added by commit aaa229529244 ("powerpc/mm: Add physical address to > Linux page table dump") [https://github.com/torvalds/linux/commit/aaa2295] > > How do other architectures deal with the problem described by the commit log of that patch ? > > Christophe
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