The flag, FAULT_FLAG_ALLOW_RETRY, was introduced by the patch,
mm: retry page fault when blocking on disk transfer
commit: d065bd810b6deb67d4897a14bfe21f8eb526ba99
for reducing mmap_sem hold times that are caused by waiting for disk
transfers when accessing file mapped VMAs.
To break COW, handle_mm_fault() is repeated with mmap_sem held, where
the introduced flag could be used again.
The straight way is to add changes in break_ksm(), but the function could be
under write-mode mmap_sem, so it has to be dupilcated.
Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
---
--- a/mm/ksm.c Sat Nov 19 16:08:10 2011
+++ b/mm/ksm.c Sat Nov 19 19:33:49 2011
@@ -394,7 +394,31 @@ static void break_cow(struct rmap_item *
goto out;
if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
goto out;
- break_ksm(vma, addr);
+ for (;;) {
+ struct page *page;
+ int ret;
+
+ page = follow_page(vma, addr, FOLL_GET);
+ if (IS_ERR_OR_NULL(page))
+ break;
+
+ if (PageKsm(page))
+ ret = handle_mm_fault(mm, vma, addr,
+ FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_WRITE);
+ else
+ ret = VM_FAULT_WRITE;
+
+ put_page(page);
+
+ if (!(ret & (VM_FAULT_WRITE|VM_FAULT_SIGBUS|VM_FAULT_OOM))) {
+ if (ret & VM_FAULT_RETRY)
+ down_read(&mm->mmap_sem);
+ } else {
+ if (ret & VM_FAULT_RETRY)
+ return;
+ break;
+ }
+ }
out:
up_read(&mm->mmap_sem);
}
--
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