Re: [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding

Christophe Leroy
2020-09-08T05:22:50Z

Le 07/09/2020 à 22:12, Mike Rapoport a écrit :
> On Mon, Sep 07, 2020 at 08:00:55PM +0200, Gerald Schaefer wrote:
>> This is v2 of an RFC previously discussed here:
>> https://lore.kernel.org/lkml/20200828140314.8556-1-gerald.schaefer@linux.ibm.com/
>>
>> Patch 1 is a fix for a regression in gup_fast on s390, after our conversion
>> to common gup_fast code. It will introduce special helper functions
>> pXd_addr_end_folded(), which have to be used in places where pagetable walk
>> is done w/o lock and with READ_ONCE, so currently only in gup_fast.
>>
>> Patch 2 is an attempt to make that more generic, i.e. change pXd_addr_end()
>> themselves by adding an extra pXd value parameter. That was suggested by
>> Jason during v1 discussion, because he is already thinking of some other
>> places where he might want to switch to the READ_ONCE logic for pagetable
>> walks. In general, that would be the cleanest / safest solution, but there
>> is some impact on other architectures and common code, hence the new and
>> greatly enlarged recipient list.
>>
>> Patch 3 is a "nice to have" add-on, which makes pXd_addr_end() inline
>> functions instead of #defines, so that we get some type checking for the
>> new pXd value parameter.
>>
>> Not sure about Fixes/stable tags for the generic solution. Only patch 1
>> fixes a real bug on s390, and has Fixes/stable tags. Patches 2 + 3 might
>> still be nice to have in stable, to ease future backports, but I guess
>> "nice to have" does not really qualify for stable backports.
>
> I also think that adding pXd parameter to pXd_addr_end() is a cleaner
> way and with this patch 1 is not really required. I would even merge
> patches 2 and 3 into a single patch and use only it as the fix.

Why not merging patches 2 and 3, but I would keep patch 1 separate but
after the generic changes, so that we first do the generic changes, then
we do the specific S390 use of it.

Christophe