mm/hugetlb: handle FOLL_DUMP well in follow_page_mask()

Patch series "mm/gup: Unify hugetlb, speed up thp", v4.

Hugetlb has a special path for slow gup that follow_page_mask() is
actually skipped completely along with faultin_page().  It's not only
confusing, but also duplicating a lot of logics that generic gup already
has, making hugetlb slightly special.

This patchset tries to dedup the logic, by first touching up the slow gup
code to be able to handle hugetlb pages correctly with the current follow
page and faultin routines (where we're mostly there..  due to 10 years ago
we did try to optimize thp, but half way done; more below), then at the
last patch drop the special path, then the hugetlb gup will always go the
generic routine too via faultin_page().

Note that hugetlb is still special for gup, mostly due to the pgtable
walking (hugetlb_walk()) that we rely on which is currently per-arch.  But
this is still one small step forward, and the diffstat might be a proof
too that this might be worthwhile.

Then for the "speed up thp" side: as a side effect, when I'm looking at
the chunk of code, I found that thp support is actually partially done. 
It doesn't mean that thp won't work for gup, but as long as **pages
pointer passed over, the optimization will be skipped too.  Patch 6 should
address that, so for thp we now get full speed gup.

For a quick number, "chrt -f 1 ./gup_test -m 512 -t -L -n 1024 -r 10"
gives me 13992.50us -> 378.50us.  Gup_test is an extreme case, but just to
show how it affects thp gups.


This patch (of 8):

Firstly, the no_page_table() is meaningless for hugetlb which is a no-op
there, because a hugetlb page always satisfies:

  - vma_is_anonymous() == false
  - vma->vm_ops->fault != NULL

So we can already safely remove it in hugetlb_follow_page_mask(), alongside
with the page* variable.

Meanwhile, what we do in follow_hugetlb_page() actually makes sense for a
dump: we try to fault in the page only if the page cache is already
allocated.  Let's do the same here for follow_page_mask() on hugetlb.

It should so far has zero effect on real dumps, because that still goes
into follow_hugetlb_page().  But this may start to influence a bit on
follow_page() users who mimics a "dump page" scenario, but hopefully in a
good way.  This also paves way for unifying the hugetlb gup-slow.

Link: https://lkml.kernel.org/r/20230628215310.73782-1-peterx@redhat.com
Link: https://lkml.kernel.org/r/20230628215310.73782-2-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A . Shutemov <kirill@shutemov.name>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/gup.c

@@ -811,7 +811,6 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 			      struct follow_page_context *ctx)
 {
 	pgd_t *pgd;
-	struct page *page;
 	struct mm_struct *mm = vma->vm_mm;
 
 	ctx->page_mask = 0;
@@ -824,12 +823,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 	 * hugetlb_follow_page_mask is only for follow_page() handling here.
 	 * Ordinary GUP uses follow_hugetlb_page for hugetlb processing.
 	 */
-	if (is_vm_hugetlb_page(vma)) {
-		page = hugetlb_follow_page_mask(vma, address, flags);
-		if (!page)
-			page = no_page_table(vma, flags);
-		return page;
-	}
+	if (is_vm_hugetlb_page(vma))
+		return hugetlb_follow_page_mask(vma, address, flags);
 
 	pgd = pgd_offset(mm, address);
 

mm/hugetlb.c

@@ -6498,6 +6498,15 @@ out:
 	spin_unlock(ptl);
 out_unlock:
 	hugetlb_vma_unlock_read(vma);
+
+	/*
+	 * Fixup retval for dump requests: if pagecache doesn't exist,
+	 * don't try to allocate a new page but just skip it.
+	 */
+	if (!page && (flags & FOLL_DUMP) &&
+	    !hugetlbfs_pagecache_present(h, vma, address))
+		page = ERR_PTR(-EFAULT);
+
 	return page;
 }