Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage()
From: Christophe Leroy
Date: Fri Apr 16 2021 - 11:18:03 EST
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@xxxxxxxxxx/
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