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Subject[PATCH 1/2 -mm] mm: mremap: dwongrade mmap_sem to read when shrinking
Date
Other than munmap, mremap might be used to shrink memory mapping too.
Use __do_munmap() to shrink mapping with downgrading mmap_sem to read.

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.

Cc: Michal Hocko <mhocko@kernel.org>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
include/linux/mm.h | 2 ++
mm/mmap.c | 4 ++--
mm/mremap.c | 16 ++++++++++++----
3 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a61ebe8..3028028 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -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);

diff --git a/mm/mmap.c b/mm/mmap.c
index 847a17d..017bcfa 100644
--- 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 5c2e185..e608e92 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -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 downgrade = false;
struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
LIST_HEAD(uf_unmap_early);
LIST_HEAD(uf_unmap);
@@ -561,12 +562,16 @@ 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
*/
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)
+ downgrade = true;
ret = addr;
goto out;
}
@@ -631,7 +636,10 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
vm_unacct_memory(charged);
locked = 0;
}
- up_write(&current->mm->mmap_sem);
+ if (downgrade)
+ 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);
--
1.8.3.1
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