Re: [PATCH v2] hugetlb: simplify hugetlb handling in follow_page_mask

Le 02/09/2022 à 21:03, Mike Kravetz a écrit :

During discussions of this series [1], it was suggested that hugetlb
handling code in follow_page_mask could be simplified.  At the beginning
of follow_page_mask, there currently is a call to follow_huge_addr which
'may' handle hugetlb pages.  ia64 is the only architecture which provides
a follow_huge_addr routine that does not return error.  Instead, at each
level of the page table a check is made for a hugetlb entry.  If a hugetlb
entry is found, a call to a routine associated with that entry is made.

Currently, there are two checks for hugetlb entries at each page table
level.  The first check is of the form:
         if (p?d_huge())
                 page = follow_huge_p?d();
the second check is of the form:
         if (is_hugepd())
                 page = follow_huge_pd().

We can replace these checks, as well as the special handling routines
such as follow_huge_p?d() and follow_huge_pd() with a single routine to
handle hugetlb vmas.

A new routine hugetlb_follow_page_mask is called for hugetlb vmas at the
beginning of follow_page_mask.  hugetlb_follow_page_mask will use the
existing routine huge_pte_offset to walk page tables looking for hugetlb
entries.  huge_pte_offset can be overwritten by architectures, and already
handles special cases such as hugepd entries.

[1] https://lore.kernel.org/linux-mm/cover.1661240170.git.baolin.wang@linux.alibaba.com/ (local)

Suggested-by: David Hildenbrand <redacted>
Signed-off-by: Mike Kravetz <redacted>
---
v2 -	Added WARN_ON_ONCE() and updated comment as suggested by David
	Fixed build issue found by kernel test robot
	Added vma (pmd sharing) locking to hugetlb_follow_page_mask
	ReBased on Baolin's patch to fix issues with CONT_* entries

  arch/ia64/mm/hugetlbpage.c    |  15 ---
  arch/powerpc/mm/hugetlbpage.c |  37 -------
  include/linux/hugetlb.h       |  51 ++--------
  mm/gup.c                      |  80 +++------------
  mm/hugetlb.c                  | 182 ++++++++++++----------------------
  5 files changed, 87 insertions(+), 278 deletions(-)

This function is called only when is_vm_hugetlb_page() is true.

When !CONFIG_HUGETLB_PAGE is_vm_hugetlb_page() always returns false, so 
the call to hugetlb_follow_page_mask() should never be compiled in.

Use BUILD_BUG() to catch it at buildtime.

+}
+
  static inline long follow_hugetlb_page(struct mm_struct *mm,
  			struct vm_area_struct *vma, struct page **pages,
  			struct vm_area_struct **vmas, unsigned long *position,