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

[Steven Price]

On 19/04/2021 14:14, 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:
> > > >
> > > > 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 ;)
>
> 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