On 19/04/2021 14:14, Christophe Leroy wrote:

Le 16/04/2021 à 12:51, Steven Price a écrit :

quoted

On 16/04/2021 11:38, Christophe Leroy wrote:

quoted

Le 16/04/2021 à 11:28, Steven Price a écrit :

quoted

On 15/04/2021 18:18, Christophe Leroy wrote:

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 ;)

Looking at how the generic ptdump code handles KASAN, I'm a bit sceptic.

IIUC, it is checking that kasan_early_shadow_pte is in the same page as 
the pgtable referred by the PMD entry. But what happens if that PMD 
entry is referring another pgtable which is inside the same page as 
kasan_early_shadow_pte ?

Shouldn't the test be

     if (pmd_page_vaddr(val) == lm_alias(kasan_early_shadow_pte))
         return note_kasan_page_table(walk, addr);

Now I come to look at this code again, I think you're right. On arm64 
this doesn't cause a problem - page tables are page sized and page 
aligned, so there couldn't be any non-KASAN pgtables sharing the page. 
Obviously that's not necessarily true of other architectures.

Feel free to add a patch to your series ;)

Steve

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