Re: [PATCH v3 00/12] mm/gup: Unify hugetlb, part 2

[Christophe Leroy]

Le 21/03/2024 à 23:07, peterx@xxxxxxxxxx a écrit :
> From: Peter Xu <peterx@xxxxxxxxxx>
> 
> v3:
> - Rebased to latest mm-unstalbe (a824831a082f, of March 21th)
> - Dropped patch to introduce pmd_thp_or_huge(), replace such uses (and also
>    pXd_huge() users) with pXd_leaf() [Jason]
> - Add a comment for CONFIG_PGTABLE_HAS_HUGE_LEAVES [Jason]
> - Use IS_ENABLED() in follow_huge_pud() [Jason]
> - Remove redundant none pud check in follow_pud_mask() [Jason]
> 
> rfc: https://lore.kernel.org/r/20231116012908.392077-1-peterx@xxxxxxxxxx
> v1:  https://lore.kernel.org/r/20231219075538.414708-1-peterx@xxxxxxxxxx
> v2:  https://lore.kernel.org/r/20240103091423.400294-1-peterx@xxxxxxxxxx
> 
> The series removes the hugetlb slow gup path after a previous refactor work
> [1], so that slow gup now uses the exact same path to process all kinds of
> memory including hugetlb.
> 
> For the long term, we may want to remove most, if not all, call sites of
> huge_pte_offset().  It'll be ideal if that API can be completely dropped
> from arch hugetlb API.  This series is one small step towards merging
> hugetlb specific codes into generic mm paths.  From that POV, this series
> removes one reference to huge_pte_offset() out of many others.
> 
> One goal of such a route is that we can reconsider merging hugetlb features
> like High Granularity Mapping (HGM).  It was not accepted in the past
> because it may add lots of hugetlb specific codes and make the mm code even
> harder to maintain.  With a merged codeset, features like HGM can hopefully
> share some code with THP, legacy (PMD+) or modern (continuous PTEs).
> 
> To make it work, the generic slow gup code will need to at least understand
> hugepd, which is already done like so in fast-gup.  Due to the specialty of
> hugepd to be software-only solution (no hardware recognizes the hugepd
> format, so it's purely artificial structures), there's chance we can merge
> some or all hugepd formats with cont_pte in the future.  That question is
> yet unsettled from Power side to have an acknowledgement.  As of now for
> this series, I kept the hugepd handling because we may still need to do so
> before getting a clearer picture of the future of hugepd.  The other reason
> is simply that we did it already for fast-gup and most codes are still
> around to be reused.  It'll make more sense to keep slow/fast gup behave
> the same before a decision is made to remove hugepd.
> 

It is not true that hugepd is a software-only solution. Powerpc 8xx HW 
matches the hugepd topology for 8M pages.

Christophe