linux-kernel - Re: [PATCH v3 12/12] mm/gup: Handle hugetlb in the generic follow_page

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Date: Fri, 22 Mar 2024 13:48:18 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: peterx@...hat.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, Michael Ellerman <mpe@...erman.id.au>,
 Christophe Leroy <christophe.leroy@...roup.eu>, Matthew Wilcox
 <willy@...radead.org>, Rik van Riel <riel@...riel.com>, Lorenzo Stoakes
 <lstoakes@...il.com>, Axel Rasmussen <axelrasmussen@...gle.com>, Yang Shi
 <shy828301@...il.com>, John Hubbard <jhubbard@...dia.com>,
 linux-arm-kernel@...ts.infradead.org, "Kirill A . Shutemov"
 <kirill@...temov.name>, Andrew Jones <andrew.jones@...ux.dev>, Vlastimil
 Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Muchun Song
 <muchun.song@...ux.dev>, Christoph Hellwig <hch@...radead.org>,
 linux-riscv@...ts.infradead.org, James Houghton <jthoughton@...gle.com>,
 David Hildenbrand <david@...hat.com>, Jason Gunthorpe <jgg@...dia.com>,
 Andrea Arcangeli <aarcange@...hat.com>, "Aneesh Kumar K . V"
 <aneesh.kumar@...nel.org>, Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v3 12/12] mm/gup: Handle hugetlb in the generic
 follow_page_mask code

On Thu, 21 Mar 2024 18:08:02 -0400 peterx@...hat.com wrote:

> From: Peter Xu <peterx@...hat.com>
> 
> Now follow_page() is ready to handle hugetlb pages in whatever form, and
> over all architectures.  Switch to the generic code path.
> 
> Time to retire hugetlb_follow_page_mask(), following the previous
> retirement of follow_hugetlb_page() in 4849807114b8.
> 
> There may be a slight difference of how the loops run when processing slow
> GUP over a large hugetlb range on cont_pte/cont_pmd supported archs: each
> loop of __get_user_pages() will resolve one pgtable entry with the patch
> applied, rather than relying on the size of hugetlb hstate, the latter may
> cover multiple entries in one loop.
> 
> A quick performance test on an aarch64 VM on M1 chip shows 15% degrade over
> a tight loop of slow gup after the path switched.  That shouldn't be a
> problem because slow-gup should not be a hot path for GUP in general: when
> page is commonly present, fast-gup will already succeed, while when the
> page is indeed missing and require a follow up page fault, the slow gup
> degrade will probably buried in the fault paths anyway.  It also explains
> why slow gup for THP used to be very slow before 57edfcfd3419 ("mm/gup:
> accelerate thp gup even for "pages != NULL"") lands, the latter not part of
> a performance analysis but a side benefit.  If the performance will be a
> concern, we can consider handle CONT_PTE in follow_page().
> 
> Before that is justified to be necessary, keep everything clean and simple.
> 

mm/gup.c:33:21: warning: 'follow_hugepd' declared 'static' but never defined [-Wunused-function]
   33 | static struct page *follow_hugepd(struct vm_area_struct *vma, hugepd_t hugepd,
      |                     ^~~~~~~~~~~~~

--- a/mm/gup.c~mm-gup-handle-hugepd-for-follow_page-fix
+++ a/mm/gup.c
@@ -30,10 +30,12 @@ struct follow_page_context {
 	unsigned int page_mask;
 };

+#ifdef CONFIG_HAVE_FAST_GUP
 static struct page *follow_hugepd(struct vm_area_struct *vma, hugepd_t hugepd,
 				  unsigned long addr, unsigned int pdshift,
 				  unsigned int flags,
 				  struct follow_page_context *ctx);
+#endif

 static inline void sanity_check_pinned_pages(struct page **pages,
 					     unsigned long npages)
_

This looks inelegant.

That's two build issues so far.  Please be more expansive in the
Kconfig variations when testing.  Especially when mucking with pgtable
macros.