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>

1 files changed, 16 insertions, 4 deletions

diff --git a/mm/mremap.c b/mm/mremap.c
index a9617e72e6b7..7f9f9180e401 100644
--- 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);