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

[Steven Price]

On 16/04/2021 16:15, Christophe Leroy wrote:
> 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.

Yes that should work. Thanks for figuring it out!

> > I see that other architectures except RISCV don't dump the physical 
> > address. But even RISCV doesn't include that check.

Yes not having the physical address certainly simplifies things - 
although I can see why that can be handy to see. The disadvantage is 
that user space or vmalloc()'d memory will produce a lot of output 
because the physical addresses are unlikely to be contiguous. And for 
most uses you don't need the information.

> > 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 ?

AFAIK other architectures are "broken" in this regard. In practice I 
don't think it often causes an issue though.

Steve