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SubjectRe: Flushing whole page instead of work for ptrace
Date
This is a VM question more than a ptrace question.  
I can't give you any authoritative answers about the VM issues.

Documentation/cachetlb.txt says:

Any time the kernel writes to a page cache page, _OR_
the kernel is about to read from a page cache page and
user space shared/writable mappings of this page potentially
exist, this routine is called.

In your case, the kernel is only reading (write=0 passed to
access_process_vm and get_user_pages). In normal situations,
the page in question will have only a private and read-only
mapping in user space. So the call should not be required in
these cases--if the code can tell that's so.

Perhaps something like the following would be safe.
But you really need some VM folks to tell you for sure.

diff --git a/mm/memory.c b/mm/memory.c
index 02e48aa..2864ee7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1484,7 +1484,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
pages[i] = page;

flush_anon_page(vma, page, start);
- flush_dcache_page(page);
+ if ((vm_flags & VM_WRITE) || (vma->vm_flags & VM_SHARED)
+ flush_dcache_page(page);
}
if (vmas)
vmas[i] = vma;

Thanks,
Roland


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