The patch titled
Subject: mm: mremap: downgrade mmap_sem to read when shrinking
has been added to the -mm tree. Its filename is
mm-mremap-downgrade-mmap_sem-to-read-when-shrinking.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-mremap-downgrade-mmap_sem-to-read-when-shrinking.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-mremap-downgrade-mmap_sem-to-read-when-shrinking.patch
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------------------------------------------------------
From: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Subject: 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.
Link: http://lkml.kernel.org/r/1538067582-60038-1-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 2 ++
mm/mmap.c | 4 ++--
mm/mremap.c | 17 +++++++++++++----
3 files changed, 17 insertions(+), 6 deletions(-)
--- a/include/linux/mm.h~mm-mremap-downgrade-mmap_sem-to-read-when-shrinking
+++ a/include/linux/mm.h
@@ -2288,6 +2288,8 @@ extern unsigned long do_mmap(struct file
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);
--- a/mm/mmap.c~mm-mremap-downgrade-mmap_sem-to-read-when-shrinking
+++ a/mm/mmap.c
@@ -2687,8 +2687,8 @@ int split_vma(struct mm_struct *mm, stru
* work. This now handles partial unmappings.
* Jeremy Fitzhardinge <jeremy@xxxxxxxx>
*/
-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;
--- a/mm/mremap.c~mm-mremap-downgrade-mmap_sem-to-read-when-shrinking
+++ a/mm/mremap.c
@@ -525,6 +525,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, a
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);
@@ -561,12 +562,17 @@ SYSCALL_DEFINE5(mremap, unsigned long, a
/*
* 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
+ * downgrade mmap_sem to read.
*/
if (old_len >= new_len) {
- ret = do_munmap(mm, addr+new_len, old_len - new_len, &uf_unmap);
- if (ret && old_len != new_len)
+ ret = __do_munmap(mm, addr+new_len, old_len - new_len,
+ &uf_unmap, true);
+ if (ret < 0 && old_len != new_len)
goto out;
+ /* Returning 1 indicates mmap_sem is downgraded to read. */
+ else if (ret == 1)
+ downgraded = true;
ret = addr;
goto out;
}
@@ -631,7 +637,10 @@ out:
vm_unacct_memory(charged);
locked = 0;
}
- up_write(¤t->mm->mmap_sem);
+ if (downgraded)
+ up_read(¤t->mm->mmap_sem);
+ else
+ up_write(¤t->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);
_
Patches currently in -mm which might be from yang.shi@xxxxxxxxxxxxxxxxx are
mm-mmap-zap-pages-with-read-mmap_sem-in-munmap.patch
mm-unmap-vm_hugetlb-mappings-with-optimized-path.patch
mm-unmap-vm_pfnmap-mappings-with-optimized-path.patch
mm-mremap-downgrade-mmap_sem-to-read-when-shrinking.patch
mm-brk-downgrade-mmap_sem-to-read-when-shrinking.patch
