flush_page_to_ram() question in kernel/ptrace.c

flush_page_to_ram() question in kernel/ptrace.c

[Manfred Spraul]

>From linux/kernel/ptrace.c, access_one_page():

>        flush_cache_page(vma, addr);
> 
>         if (write) {
>                 maddr = kmap(page);
>                 memcpy(maddr + (addr & ~PAGE_MASK), buf, len);
>                 flush_page_to_ram(page);
>                 flush_icache_page(vma, page);
>                 kunmap(page);
>         } else {
>                 maddr = kmap(page);
>                 memcpy(buf, maddr + (addr & ~PAGE_MASK), len);
>                 flush_page_to_ram(page);
                  ^^^^^^^^^^^^^^^^^^^^^^
>                 kunmap(page);
>         }

Is this flush required?

The memcpy read from the mapping, it didn't write.

--
	Manfred

Re: flush_page_to_ram() question in kernel/ptrace.c

[David S. Miller]

Manfred Spraul writes:
 > >                 memcpy(buf, maddr + (addr & ~PAGE_MASK), len);
 > >                 flush_page_to_ram(page);
 >                   ^^^^^^^^^^^^^^^^^^^^^^
 > Is this flush required?
 > 
 > The memcpy read from the mapping, it didn't write.

You have to kick it out of the cache so that future reads on the
kernel side don't get stale data with caching setups that allow
illegal aliases to form.

Later,
David S. Miller
davem@redhat.com