mm/munlock: mlock_pte_range() when mlocking or munlocking

Fill in missing pieces: reimplementation of munlock_vma_pages_range(),
required to lower the mlock_counts when munlocking without munmapping;
and its complement, implementation of mlock_vma_pages_range(), required
to raise the mlock_counts on pages already there when a range is mlocked.

Combine them into just the one function mlock_vma_pages_range(), using
walk_page_range() to run mlock_pte_range().  This approach fixes the
"Very slow unlockall()" of unpopulated PROT_NONE areas, reported in
https://lore.kernel.org/linux-mm/70885d37-62b7-748b-29df-9e94f3291736@gmail.com/

Munlock clears VM_LOCKED at the start, under exclusive mmap_lock; but if
a racing truncate or holepunch (depending on i_mmap_rwsem) gets to the
pte first, it will not try to munlock the page: leaving release_pages()
to correct it when the last reference to the page is gone - that's okay,
a page is not evictable anyway while it is held by an extra reference.

Mlock sets VM_LOCKED at the start, under exclusive mmap_lock; but if
a racing remove_migration_pte() or try_to_unmap_one() (depending on
i_mmap_rwsem) gets to the pte first, it will try to mlock the page,
then mlock_pte_range() mlock it a second time.  This is harder to
reproduce, but a more serious race because it could leave the page
unevictable indefinitely though the area is munlocked afterwards.
Guard against it by setting the (inappropriate) VM_IO flag,
and modifying mlock_vma_page() to decline such vmas.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

diff --git a/mm/internal.h b/mm/internal.h
index a43d79335c1633b1f13a2d4311361acb35b0d9af..b3f0dd3ffba2b21b0d5547d13d9f8e720686a8e8 100644 (file)
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -412,7 +412,8 @@ void mlock_page(struct page *page);
 static inline void mlock_vma_page(struct page *page,
                        struct vm_area_struct *vma, bool compound)
 {
-       if (unlikely(vma->vm_flags & VM_LOCKED) &&
+       /* VM_IO check prevents migration from double-counting during mlock */
+       if (unlikely((vma->vm_flags & (VM_LOCKED|VM_IO)) == VM_LOCKED) &&
            (compound || !PageTransCompound(page)))
                mlock_page(page);
 }

diff --git a/mm/mlock.c b/mm/mlock.c
index f8a3a54687dd619f6081a5255b0472517e4e864f..581ea8bf1b83c7a714b5d83315cb25af08084138 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -14,6 +14,7 @@
 #include <linux/swapops.h>
 #include <linux/pagemap.h>
 #include <linux/pagevec.h>
+#include <linux/pagewalk.h>
 #include <linux/mempolicy.h>
 #include <linux/syscalls.h>
 #include <linux/sched.h>
@@ -127,25 +128,91 @@ out:
        unlock_page_memcg(page);
 }
 
+static int mlock_pte_range(pmd_t *pmd, unsigned long addr,
+                          unsigned long end, struct mm_walk *walk)
+
+{
+       struct vm_area_struct *vma = walk->vma;
+       spinlock_t *ptl;
+       pte_t *start_pte, *pte;
+       struct page *page;
+
+       ptl = pmd_trans_huge_lock(pmd, vma);
+       if (ptl) {
+               if (!pmd_present(*pmd))
+                       goto out;
+               if (is_huge_zero_pmd(*pmd))
+                       goto out;
+               page = pmd_page(*pmd);
+               if (vma->vm_flags & VM_LOCKED)
+                       mlock_page(page);
+               else
+                       munlock_page(page);
+               goto out;
+       }
+
+       start_pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
+       for (pte = start_pte; addr != end; pte++, addr += PAGE_SIZE) {
+               if (!pte_present(*pte))
+                       continue;
+               page = vm_normal_page(vma, addr, *pte);
+               if (!page)
+                       continue;
+               if (PageTransCompound(page))
+                       continue;
+               if (vma->vm_flags & VM_LOCKED)
+                       mlock_page(page);
+               else
+                       munlock_page(page);
+       }
+       pte_unmap(start_pte);
+out:
+       spin_unlock(ptl);
+       cond_resched();
+       return 0;
+}
+
 /*
- * munlock_vma_pages_range() - munlock all pages in the vma range.'
- * @vma - vma containing range to be munlock()ed.
+ * mlock_vma_pages_range() - mlock any pages already in the range,
+ *                           or munlock all pages in the range.
+ * @vma - vma containing range to be mlock()ed or munlock()ed
  * @start - start address in @vma of the range
- * @end - end of range in @vma.
- *
- *  For mremap(), munmap() and exit().
+ * @end - end of range in @vma
+ * @newflags - the new set of flags for @vma.
  *
- * Called with @vma VM_LOCKED.
- *
- * Returns with VM_LOCKED cleared.  Callers must be prepared to
- * deal with this.
+ * Called for mlock(), mlock2() and mlockall(), to set @vma VM_LOCKED;
+ * called for munlock() and munlockall(), to clear VM_LOCKED from @vma.
  */
-static void munlock_vma_pages_range(struct vm_area_struct *vma,
-                                   unsigned long start, unsigned long end)
+static void mlock_vma_pages_range(struct vm_area_struct *vma,
+       unsigned long start, unsigned long end, vm_flags_t newflags)
 {
-       vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
+       static const struct mm_walk_ops mlock_walk_ops = {
+               .pmd_entry = mlock_pte_range,
+       };
 
-       /* Reimplementation to follow in later commit */
+       /*
+        * There is a slight chance that concurrent page migration,
+        * or page reclaim finding a page of this now-VM_LOCKED vma,
+        * will call mlock_vma_page() and raise page's mlock_count:
+        * double counting, leaving the page unevictable indefinitely.
+        * Communicate this danger to mlock_vma_page() with VM_IO,
+        * which is a VM_SPECIAL flag not allowed on VM_LOCKED vmas.
+        * mmap_lock is held in write mode here, so this weird
+        * combination should not be visible to other mmap_lock users;
+        * but WRITE_ONCE so rmap walkers must see VM_IO if VM_LOCKED.
+        */
+       if (newflags & VM_LOCKED)
+               newflags |= VM_IO;
+       WRITE_ONCE(vma->vm_flags, newflags);
+
+       lru_add_drain();
+       walk_page_range(vma->vm_mm, start, end, &mlock_walk_ops, NULL);
+       lru_add_drain();
+
+       if (newflags & VM_IO) {
+               newflags &= ~VM_IO;
+               WRITE_ONCE(vma->vm_flags, newflags);
+       }
 }
 
 /*
@@ -164,10 +231,9 @@ static int mlock_fixup(struct vm_area_struct *vma, struct vm_area_struct **prev,
        pgoff_t pgoff;
        int nr_pages;
        int ret = 0;
-       int lock = !!(newflags & VM_LOCKED);
-       vm_flags_t old_flags = vma->vm_flags;
+       vm_flags_t oldflags = vma->vm_flags;
 
-       if (newflags == vma->vm_flags || (vma->vm_flags & VM_SPECIAL) ||
+       if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
            is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
            vma_is_dax(vma) || vma_is_secretmem(vma))
                /* don't set VM_LOCKED or VM_LOCKONFAULT and don't count */
@@ -199,9 +265,9 @@ success:
         * Keep track of amount of locked VM.
         */
        nr_pages = (end - start) >> PAGE_SHIFT;
-       if (!lock)
+       if (!(newflags & VM_LOCKED))
                nr_pages = -nr_pages;
-       else if (old_flags & VM_LOCKED)
+       else if (oldflags & VM_LOCKED)
                nr_pages = 0;
        mm->locked_vm += nr_pages;
 
@@ -211,11 +277,12 @@ success:
         * set VM_LOCKED, populate_vma_page_range will bring it back.
         */
 
-       if (lock)
+       if ((newflags & VM_LOCKED) && (oldflags & VM_LOCKED)) {
+               /* No work to do, and mlocking twice would be wrong */
                vma->vm_flags = newflags;
-       else
-               munlock_vma_pages_range(vma, start, end);
-
+       } else {
+               mlock_vma_pages_range(vma, start, end, newflags);
+       }
 out:
        *prev = vma;
        return ret;