Re: [PATCH RFC 0/6] mm: THP-agnostic refactor on huge mappings

[David Hildenbrand <david@redhat.com>]

On 18.07.24 00:02, Peter Xu wrote:
> This is an RFC series, so not yet for merging.  Please don't be scared by
> the code changes: most of them are code movements only.
> 
> This series is based on the dax mprotect fix series here (while that one is
> based on mm-unstable):
> 
>    [PATCH v3 0/8] mm/mprotect: Fix dax puds
>    https://lore.kernel.org/r/20240715192142.3241557-1-peterx@redhat.com
> 
> Overview
> ========
> 
> This series doesn't provide any feature change.  The only goal of this
> series is to start decoupling two ideas: "THP" and "huge mapping".  We
> already started with having PGTABLE_HAS_HUGE_LEAVES config option, and this
> one extends that idea into the code.
> 
> The issue is that we have so many functions that only compile with
> CONFIG_THP=on, even though they're about huge mappings, and huge mapping is
> a pretty common concept, which can apply to many things besides THPs
> nowadays.  The major THP file is mm/huge_memory.c as of now.
> 
> The first example of such huge mapping users will be hugetlb.  We lived
> until now with no problem simply because Linux almost duplicated all the
> logics there in the "THP" files into hugetlb APIs.  If we want to get rid
> of hugetlb specific APIs and paths, this _might_ be the first thing we want
> to do, because we want to be able to e.g., zapping a hugetlb pmd entry even
> if !CONFIG_THP.
> 
> Then consider other things like dax / pfnmaps.  Dax can depend on THP, then
> it'll naturally be able to use pmd/pud helpers, that's okay.  However is it
> a must?  Do we also want to have every new pmd/pud mappings in the future
> to depend on THP (like PFNMAP)?  My answer is no, but I'm open to opinions.
> 
> If anyone agrees with me that "huge mapping" (aka, PMD/PUD mappings that
> are larger than PAGE_SIZE) is a more generic concept than THP, then I think
> at some point we need to move the generic code out of THP code into a
> common code base.
> 
> This is what this series does as a start.

Hi Peter!

 From a quick glimpse, patch #1-#4 do make sense independent of patch #5.

I am not so sure about all of the code movement in patch #5. If large 
folios are the future, then likely huge_memory.c should simply be the 
home for all that logic.

Maybe the goal should better be to compile huge_memory.c not only for 
THP, but also for other use cases that require that logic, and fence off 
all THP specific stuff using #ifdef?

Not sure, though. But a lot of this code movements/churn might be avoidable.

-- 
Cheers,

David / dhildenb