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authorYang Shi <yang.shi@linux.alibaba.com>2018-10-26 15:08:50 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2018-10-26 16:26:35 -0700
commit85a06835f6f1ba79f0f00838ccd5ad840dd1eafb (patch)
tree91ec31587d1e465de5d5940a11f469695c7320a3 /mm/mremap.c
parent3c0513243a4a07ebad2d59f3d972bef483818ec6 (diff)
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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>
Diffstat (limited to 'mm/mremap.c')
-rw-r--r--mm/mremap.c20
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);