@@ -2286,6 +2286,8 @@ 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);
@@ -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;
@@ -525,6 +525,7 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
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 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
/*
* 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 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
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);
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. 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: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> --- v3: Fixed the comments from Vlastimil and Kirill. And, added their Acked-by. Thanks. v2: Rephrase the commit log per Michal include/linux/mm.h | 2 ++ mm/mmap.c | 4 ++-- mm/mremap.c | 17 +++++++++++++---- 3 files changed, 17 insertions(+), 6 deletions(-)