Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage()
From: Steven Price
Date: Fri Apr 16 2021 - 09:00:38 EST
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@xxxxxxxxxx>
---
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@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.
Steve