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

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Date: Fri, 26 Apr 2024 09:44:58 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, 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 Fri, Apr 26, 2024 at 09:17:47AM +0200, David Hildenbrand wrote:
> 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 ...

Yes it's not needed, just doesn't look worthwhile to put another helper on
top just for this.  I mentioned this in the commit message here:

  There's something not needed for follow page, for example, gup_hugepte()
  tries to detect pgtable entry change which will never happen with slow
  gup (which has the pgtable lock held), but that's not a problem to check.

> 
> @Peter, any insights?

However I think we should pass vma in for sure, I guess I overlooked that,
and it didn't expose in my tests too as I probably missed ./cow.

I'll prepare a separate patch on top of this series and the gup-fast rename
patches (I saw this one just reached mm-stable), and I'll see whether I can
test it too if I can find a Power system fast enough.  I'll probably drop
the "fast" in the hugepd function names too.

Thanks,

-- 
Peter Xu