Re: [PATCH v1 1/3] mm/gup: consistently name GUP-fast functions

David Hildenbrand
2024-04-26T07:17:54Z

On 02.04.24 14:55, David Hildenbrand wrote:
> Let's consistently call the "fast-only" part of GUP "GUP-fast" and rename
> all relevant internal functions to start with "gup_fast", to make it
> clearer that this is not ordinary GUP. The current mixture of
> "lockless", "gup" and "gup_fast" is confusing.
>
> Further, avoid the term "huge" when talking about a "leaf" -- for
> example, we nowadays check pmd_leaf() because pmd_huge() is gone. For the
> "hugepd"/"hugepte" stuff, it's part of the name ("is_hugepd"), so that
> stays.
>
> What remains is the "external" interface:
> * get_user_pages_fast_only()
> * get_user_pages_fast()
> * pin_user_pages_fast()
>
> The high-level internal functions for GUP-fast (+slow fallback) are now:
> * internal_get_user_pages_fast() -> gup_fast_fallback()
> * lockless_pages_from_mm() -> gup_fast()
>
> The basic GUP-fast walker functions:
> * gup_pgd_range() -> gup_fast_pgd_range()
> * gup_p4d_range() -> gup_fast_p4d_range()
> * gup_pud_range() -> gup_fast_pud_range()
> * gup_pmd_range() -> gup_fast_pmd_range()
> * gup_pte_range() -> gup_fast_pte_range()
> * gup_huge_pgd() -> gup_fast_pgd_leaf()
> * gup_huge_pud() -> gup_fast_pud_leaf()
> * gup_huge_pmd() -> gup_fast_pmd_leaf()
>
> The weird hugepd stuff:
> * gup_huge_pd() -> gup_fast_hugepd()
> * gup_hugepte() -> gup_fast_hugepte()

I just realized that we end up calling these from follow_hugepd() as
well. And something seems to be off, because gup_fast_hugepd() won't
have the VMA even in the slow-GUP case to pass it to gup_must_unshare().

So these are GUP-fast functions and the terminology seem correct. But
the usage from follow_hugepd() is questionable,

commit a12083d721d703f985f4403d6b333cc449f838f6
Author: Peter Xu <peterx@redhat.com>
Date: Wed Mar 27 11:23:31 2024 -0400

mm/gup: handle hugepd for follow_page()

states "With previous refactors on fast-gup gup_huge_pd(), most of the
code can be leveraged", which doesn't look quite true just staring the
the gup_must_unshare() call where we don't pass the VMA. Also,
"unlikely(pte_val(pte) != pte_val(ptep_get(ptep)" doesn't make any sense
for slow GUP ...

@Peter, any insights?

--
Cheers,

David / dhildenb