Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

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

Le 08/09/2020 à 14:09, Christian Borntraeger a écrit :
> 
> 
> On 08.09.20 07:06, Christophe Leroy wrote:
>>
>>
>> Le 07/09/2020 à 20:00, Gerald Schaefer a écrit :
>>> From: Alexander Gordeev <agordeev@linux.ibm.com>
>>>
>>> Commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast
>>> code") introduced a subtle but severe bug on s390 with gup_fast, due to
>>> dynamic page table folding.
>>>
>>> The question "What would it require for the generic code to work for s390"
>>> has already been discussed here
>>> https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1
>>> and ended with a promising approach here
>>> https://lkml.kernel.org/r/20190419153307.4f2911b5@mschwideX1
>>> which in the end unfortunately didn't quite work completely.
>>>
>>> We tried to mimic static level folding by changing pgd_offset to always
>>> calculate top level page table offset, and do nothing in folded pXd_offset.
>>> What has been overlooked is that PxD_SIZE/MASK and thus pXd_addr_end do
>>> not reflect this dynamic behaviour, and still act like static 5-level
>>> page tables.
>>>
>>
>> [...]
>>
>>>
>>> Fix this by introducing new pXd_addr_end_folded helpers, which take an
>>> additional pXd entry value parameter, that can be used on s390
>>> to determine the correct page table level and return corresponding
>>> end / boundary. With that, the pointer iteration will always
>>> happen in gup_pgd_range for s390. No change for other architectures
>>> introduced.
>>
>> Not sure pXd_addr_end_folded() is the best understandable name, allthough I don't have any alternative suggestion at the moment.
>> Maybe could be something like pXd_addr_end_fixup() as it will disappear in the next patch, or pXd_addr_end_gup() ?
>>
>> Also, if it happens to be acceptable to get patch 2 in stable, I think you should switch patch 1 and patch 2 to avoid the step through pXd_addr_end_folded()
> 
> given that this fixes a data corruption issue, wouldnt it be the best to go forward
> with this patch ASAP and then handle the other patches on top with all the time that
> we need?

I have no strong opinion on this, but I feel rather tricky to have to 
change generic part of GUP to use a new fonction then revert that change 
in the following patch, just because you want the first patch in stable 
and not the second one.

Regardless, I was wondering, why do we need a reference to the pXd at 
all when calling pXd_addr_end() ?

Couldn't S390 retrieve the pXd by using the pXd_offset() dance with the 
passed addr ?

Christophe