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 17:00:22 +0100 |
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On 16/04/2021 16:15, Christophe Leroy wrote: > > > 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.
Yes that should work. Thanks for figuring it out!
> >> >> I see that other architectures except RISCV don't dump the physical >> address. But even RISCV doesn't include that check.
Yes not having the physical address certainly simplifies things - although I can see why that can be handy to see. The disadvantage is that user space or vmalloc()'d memory will produce a lot of output because the physical addresses are unlikely to be contiguous. And for most uses you don't need the information.
>> 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 ?
AFAIK other architectures are "broken" in this regard. In practice I don't think it often causes an issue though.
Steve
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