[PATCH v2 13/13] mm/gup: Handle hugetlb in the generic follow_page_mask code

[peterx at redhat.com]

From: Peter Xu <peterx at redhat.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.

Signed-off-by: Peter Xu <peterx at redhat.com>
---
 include/linux/hugetlb.h |  7 ----
 mm/gup.c                | 15 +++------
 mm/hugetlb.c            | 71 -----------------------------------------
 3 files changed, 5 insertions(+), 88 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index e8eddd51fc17..cdbb53407722 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -332,13 +332,6 @@ static inline void hugetlb_zap_end(
 {
 }
 
-static inline struct page *hugetlb_follow_page_mask(
-    struct vm_area_struct *vma, unsigned long address, unsigned int flags,
-    unsigned int *page_mask)
-{
-	BUILD_BUG(); /* should never be compiled in if !CONFIG_HUGETLB_PAGE*/
-}
-
 static inline int copy_hugetlb_page_range(struct mm_struct *dst,
 					  struct mm_struct *src,
 					  struct vm_area_struct *dst_vma,
diff --git a/mm/gup.c b/mm/gup.c
index 245214b64108..4f8a3dc287c9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -997,18 +997,11 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 {
 	pgd_t *pgd, pgdval;
 	struct mm_struct *mm = vma->vm_mm;
+	struct page *page;
 
-	ctx->page_mask = 0;
-
-	/*
-	 * Call hugetlb_follow_page_mask for hugetlb vmas as it will use
-	 * special hugetlb page table walking code.  This eliminates the
-	 * need to check for hugetlb entries in the general walking code.
-	 */
-	if (is_vm_hugetlb_page(vma))
-		return hugetlb_follow_page_mask(vma, address, flags,
-						&ctx->page_mask);
+	vma_pgtable_walk_begin(vma);
 
+	ctx->page_mask = 0;
 	pgd = pgd_offset(mm, address);
 	pgdval = *pgd;
 
@@ -1020,6 +1013,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 	else
 		page = follow_p4d_mask(vma, address, pgd, flags, ctx);
 
+	vma_pgtable_walk_end(vma);
+
 	return page;
 }
 
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bfb52bb8b943..e13b4e038c2c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6782,77 +6782,6 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
 }
 #endif /* CONFIG_USERFAULTFD */
 
-struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
-				      unsigned long address, unsigned int flags,
-				      unsigned int *page_mask)
-{
-	struct hstate *h = hstate_vma(vma);
-	struct mm_struct *mm = vma->vm_mm;
-	unsigned long haddr = address & huge_page_mask(h);
-	struct page *page = NULL;
-	spinlock_t *ptl;
-	pte_t *pte, entry;
-	int ret;
-
-	hugetlb_vma_lock_read(vma);
-	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
-	if (!pte)
-		goto out_unlock;
-
-	ptl = huge_pte_lock(h, mm, pte);
-	entry = huge_ptep_get(pte);
-	if (pte_present(entry)) {
-		page = pte_page(entry);
-
-		if (!huge_pte_write(entry)) {
-			if (flags & FOLL_WRITE) {
-				page = NULL;
-				goto out;
-			}
-
-			if (gup_must_unshare(vma, flags, page)) {
-				/* Tell the caller to do unsharing */
-				page = ERR_PTR(-EMLINK);
-				goto out;
-			}
-		}
-
-		page = nth_page(page, ((address & ~huge_page_mask(h)) >> PAGE_SHIFT));
-
-		/*
-		 * Note that page may be a sub-page, and with vmemmap
-		 * optimizations the page struct may be read only.
-		 * try_grab_page() will increase the ref count on the
-		 * head page, so this will be OK.
-		 *
-		 * try_grab_page() should always be able to get the page here,
-		 * because we hold the ptl lock and have verified pte_present().
-		 */
-		ret = try_grab_page(page, flags);
-
-		if (WARN_ON_ONCE(ret)) {
-			page = ERR_PTR(ret);
-			goto out;
-		}
-
-		*page_mask = (1U << huge_page_order(h)) - 1;
-	}
-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;
-}
-
 long hugetlb_change_protection(struct vm_area_struct *vma,
 		unsigned long address, unsigned long end,
 		pgprot_t newprot, unsigned long cp_flags)
-- 
2.41.0