Re: [Regression 5.7-rc1] Random hangs on 32-bit PowerPC (PowerBook6,7)

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 20/05/2020 à 15:43, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> 
>> Le 18/05/2020 à 17:19, Rui Salvaterra a écrit :
>>> Hi again, Christophe,
>>>
>>> On Mon, 18 May 2020 at 15:03, Christophe Leroy
>>> <christophe.leroy@csgroup.eu> wrote:
>>>>
>>>> Can you try reverting 697ece78f8f749aeea40f2711389901f0974017a ? It may
>>>> have broken swap.
>>>
>>> Yeah, that was a good call. :) Linux 5.7-rc1 with the revert on top
>>> survives the beating. I'll be happy to test a definitive patch!
>>>
>>
>> Yeah I discovered recently that the way swap is implemented on powerpc
>> expects RW and other important bits not be one of the 3 least
>> significant bits (see __pte_to_swp_entry() )
> 
> The last 3 bits are there to track the _PAGE_PRESENT right? What is the
> RW dependency there? Are you suggesting of read/write migration entry?
> A swap entry should not retain the pte rw bits right?
> 
> A swap entry is built using swap type + offset. And it should not have a
> dependency on pte RW bits. Along with type and offset we also should
> have the ability to mark it as a pte entry and also set not present
> bits. With that understanding what am I missing here?

That's probably me who is missing something, I have not digged into the 
swap functionning yet indeed, so that was only my first feeling.

By the way, the problems is definitely due to the order changes in the 
PTE bits, whether that's because _PAGE_RW was moved to the last 3 bits 
or whether that's because _PAGE_PRESENT was moved out of the last 3 
bits, I don't know yet.

My (bad) understanding is from the fact that  __pte_to_swp_entry() is a 
right shift by 3 bits, so it looses the last 3 bits, and therefore 
__swp_entry_to_pte(__pte_to_swp_entry(pte)) looses the last 3 bits of a PTE.

Is there somewhere a description of how swap works exactly ?

Christophe