Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage()

From: Christophe Leroy
Date: Fri Apr 16 2021 - 07:08:32 EST

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 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 which uses page size to detect whether it is a KASAN like stuff.

Then came 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

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 .