Re: Flushing whole page instead of work for ptrace

From: Oleg Nesterov
Date: Fri Dec 03 2010 - 10:07:42 EST


On 11/30, Roland McGrath wrote:
>
> 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);

First of all, I know absolutely nothing about D-cache aliasing.
My poor understanding of flush_dcache_page() is: synchronize the
kernel/user vision of this memory, in the case when either side
can change it.

If this is true, then this change doesn't look right in general.

Even if (vma->vm_flags & VM_SHARED) == 0, it is possible that
tsk can write to this memory, this mapping can be writable and
private.

Even if we ensure that this mapping is readonly/private, another
user-space process can write to this page via shared/writable
mapping.


I'd like to know if my understanding is correct, I am just curious.

Oleg.

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