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

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Date: Fri, 26 Apr 2024 09:17:47 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Mike Rapoport <rppt@...nel.org>, Jason Gunthorpe <jgg@...dia.com>,
 John Hubbard <jhubbard@...dia.com>, linux-arm-kernel@...ts.infradead.org,
 loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
 linux-sh@...r.kernel.org, linux-perf-users@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-riscv@...ts.infradead.org,
 x86@...nel.org
Subject: Re: [PATCH v1 1/3] mm/gup: consistently name GUP-fast functions

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@...hat.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