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

Le 16/04/2021 à 17:04, Christophe Leroy a écrit :

quoted

Le 16/04/2021 à 16:40, Christophe Leroy a écrit :

quoted

Le 16/04/2021 à 15:00, Steven Price a écrit :

quoted

On 16/04/2021 12:08, Christophe Leroy wrote:

quoted

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

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 ;)

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

...

quoted

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!

quoted

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.

quoted

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