mm: follow_hugetlb_page flags

follow_hugetlb_page() shouldn't be guessing about the coredump case
either: pass the foll_flags down to it, instead of just the write bit.

Remove that obscure huge_zeropage_ok() test.  The decision is easy,
though unlike the non-huge case - here vm_ops->fault is always set.
But we know that a fault would serve up zeroes, unless there's
already a hugetlbfs pagecache page to back the range.

(Alternatively, since hugetlb pages aren't swapped out under pressure,
you could save more dump space by arguing that a page not yet faulted
into this process cannot be relevant to the dump; but that would be
more surprising.)

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 37 insertions, 25 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c001f846f17d..6b41f70bbc7f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2016,6 +2016,23 @@ static struct page *hugetlbfs_pagecache_page(struct hstate *h,
         return find_lock_page(mapping, idx);
 }
 
+/* Return whether there is a pagecache page to back given address within VMA */
+static bool hugetlbfs_backed(struct hstate *h,
+                        struct vm_area_struct *vma, unsigned long address)
+{
+        struct address_space *mapping;
+        pgoff_t idx;
+        struct page *page;
+
+        mapping = vma->vm_file->f_mapping;
+        idx = vma_hugecache_offset(h, vma, address);
+
+        page = find_get_page(mapping, idx);
+        if (page)
+                put_page(page);
+        return page != NULL;
+}
+
 static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
                         unsigned long address, pte_t *ptep, unsigned int flags)
 {
@@ -2211,54 +2228,52 @@ follow_huge_pud(struct mm_struct *mm, unsigned long address,
         return NULL;
 }
 
-static int huge_zeropage_ok(pte_t *ptep, int write, int shared)
-{
-        if (!ptep || write || shared)
-                return 0;
-        else
-                return huge_pte_none(huge_ptep_get(ptep));
-}
-
 int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
                         struct page **pages, struct vm_area_struct **vmas,
                         unsigned long *position, int *length, int i,
-                        int write)
+                        unsigned int flags)
 {
         unsigned long pfn_offset;
         unsigned long vaddr = *position;
         int remainder = *length;
         struct hstate *h = hstate_vma(vma);
-        int zeropage_ok = 0;
-        int shared = vma->vm_flags & VM_SHARED;
 
         spin_lock(&mm->page_table_lock);
         while (vaddr < vma->vm_end && remainder) {
                 pte_t *pte;
+                int absent;
                 struct page *page;
 
                 /*
                  * Some archs (sparc64, sh*) have multiple pte_ts to
-                 * each hugepage.  We have to make * sure we get the
+                 * each hugepage.  We have to make sure we get the
                  * first, for the page indexing below to work.
                  */
                 pte = huge_pte_offset(mm, vaddr & huge_page_mask(h));
-                if (huge_zeropage_ok(pte, write, shared))
-                        zeropage_ok = 1;
+                absent = !pte || huge_pte_none(huge_ptep_get(pte));
+
+                /*
+                 * When coredumping, it suits get_dump_page if we just return
+                 * an error if there's a hole and no huge pagecache to back it.
+                 */
+                if (absent &&
+                    ((flags & FOLL_DUMP) && !hugetlbfs_backed(h, vma, vaddr))) {
+                        remainder = 0;
+                        break;
+                }
 
-                if (!pte ||
-                    (huge_pte_none(huge_ptep_get(pte)) && !zeropage_ok) ||
-                    (write && !pte_write(huge_ptep_get(pte)))) {
+                if (absent ||
+                    ((flags & FOLL_WRITE) && !pte_write(huge_ptep_get(pte)))) {
                         int ret;
 
                         spin_unlock(&mm->page_table_lock);
-                        ret = hugetlb_fault(mm, vma, vaddr, write);
+                        ret = hugetlb_fault(mm, vma, vaddr,
+                                (flags & FOLL_WRITE) ? FAULT_FLAG_WRITE : 0);
                         spin_lock(&mm->page_table_lock);
                         if (!(ret & VM_FAULT_ERROR))
                                 continue;
 
                         remainder = 0;
-                        if (!i)
-                                i = -EFAULT;
                         break;
                 }
 
@@ -2266,10 +2281,7 @@ int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
                 page = pte_page(huge_ptep_get(pte));
 same_page:
                 if (pages) {
-                        if (zeropage_ok)
-                                pages[i] = ZERO_PAGE(0);
-                        else
-                                pages[i] = mem_map_offset(page, pfn_offset);
+                        pages[i] = mem_map_offset(page, pfn_offset);
                         get_page(pages[i]);
                 }
 
@@ -2293,7 +2305,7 @@ same_page:
         *length = remainder;
         *position = vaddr;
 
-        return i;
+        return i ? i : -EFAULT;
 }
 
 void hugetlb_change_protection(struct vm_area_struct *vma,