mm: mremap: downgrade mmap_sem to read when shrinking

Other than munmap, mremap might be used to shrink memory mapping too.
So, it may hold write mmap_sem for long time when shrinking large
mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in
munmap") described.

The mremap() will not manipulate vmas anymore after __do_munmap() call for
the mapping shrink use case, so it is safe to downgrade to read mmap_sem.

So, the same optimization, which downgrades mmap_sem to read for zapping
pages, is also feasible and reasonable to this case.

The period of holding exclusive mmap_sem for shrinking large mapping
would be reduced significantly with this optimization.

MREMAP_FIXED and MREMAP_MAYMOVE are more complicated to adopt this
optimization since they need manipulate vmas after do_munmap(),
downgrading mmap_sem may create race window.

Simple mapping shrink is the low hanging fruit, and it may cover the
most cases of unmap with munmap together.

[akpm@linux-foundation.org: tweak comment]
[yang.shi@linux.alibaba.com: fix unsigned compare against 0 issue]
Link: http://lkml.kernel.org/r/1538687672-17795-2-git-send-email-yang.shi@linux.alibaba.com
Link: http://lkml.kernel.org/r/1538067582-60038-1-git-send-email-yang.shi@linux.alibaba.com
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 33228a4..a023c5c 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2306,6 +2306,8 @@ extern unsigned long do_mmap(struct file *file, unsigned long addr,
        unsigned long len, unsigned long prot, unsigned long flags,
        vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate,
        struct list_head *uf);
+extern int __do_munmap(struct mm_struct *, unsigned long, size_t,
+                      struct list_head *uf, bool downgrade);
 extern int do_munmap(struct mm_struct *, unsigned long, size_t,
                     struct list_head *uf);
 

diff --git a/mm/mmap.c b/mm/mmap.c
index 58e323c..1bfd120 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2687,8 +2687,8 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
  * work.  This now handles partial unmappings.
  * Jeremy Fitzhardinge <jeremy@goop.org>
  */
-static int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
-                      struct list_head *uf, bool downgrade)
+int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
+               struct list_head *uf, bool downgrade)
 {
        unsigned long end;
        struct vm_area_struct *vma, *prev, *last;

diff --git a/mm/mremap.c b/mm/mremap.c
index a9617e7..7f9f918 100644 (file)
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -521,6 +521,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
        unsigned long ret = -EINVAL;
        unsigned long charged = 0;
        bool locked = false;
+       bool downgraded = false;
        struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
        LIST_HEAD(uf_unmap_early);
        LIST_HEAD(uf_unmap);
@@ -557,12 +558,20 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
        /*
         * Always allow a shrinking remap: that just unmaps
         * the unnecessary pages..
-        * do_munmap does all the needed commit accounting
+        * __do_munmap does all the needed commit accounting, and
+        * downgrades mmap_sem to read if so directed.
         */
        if (old_len >= new_len) {
-               ret = do_munmap(mm, addr+new_len, old_len - new_len, &uf_unmap);
-               if (ret && old_len != new_len)
+               int retval;
+
+               retval = __do_munmap(mm, addr+new_len, old_len - new_len,
+                                 &uf_unmap, true);
+               if (retval < 0 && old_len != new_len) {
+                       ret = retval;
                        goto out;
+               /* Returning 1 indicates mmap_sem is downgraded to read. */
+               } else if (retval == 1)
+                       downgraded = true;
                ret = addr;
                goto out;
        }
@@ -627,7 +636,10 @@ out:
                vm_unacct_memory(charged);
                locked = 0;
        }
-       up_write(&current->mm->mmap_sem);
+       if (downgraded)
+               up_read(&current->mm->mmap_sem);
+       else
+               up_write(&current->mm->mmap_sem);
        if (locked && new_len > old_len)
                mm_populate(new_addr + old_len, new_len - old_len);
        userfaultfd_unmap_complete(mm, &uf_unmap_early);