Re: [PATCH] Describe race of direct read and fork for unaligned buffers

[Nick Piggin <npiggin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 2 May 2012 18:17, Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org> wrote:
> On Wed 02-05-12 01:50:46, Nick Piggin wrote:

>> KOSAKI-san is correct, I think.
>>
>> The race is something like this:
>>
>> DIO-read
>>     page = get_user_pages()
>>                                                         fork()
>>                                                             COW(page)
>>                                                          touch(page)
>>     DMA(page)
>>     page_cache_release(page);
>>
>> So whether parent or child touches the page, determines who gets the
>> actual DMA target, and who gets the copy.
>  OK, this is roughly what I understood from original threads as well. So
> if our buffer is page aligned and its size is page aligned, you would hit
> the corruption only if you do modify the buffer while IO to / from that buffer
> is in progress. And that would seem like a really bad programming practice
> anyway. So I still believe that having everything page size aligned will
> effectively remove the problem although I agree it does not aim at the core
> of it.

I see what you mean.

I'm not sure, though. For most apps it's bad practice I think. If you get into
realm of sophisticated, performance critical IO/storage managers, it would
not surprise me if such concurrent buffer modifications could be allowed.
We allow exactly such a thing in our pagecache layer. Although probably
those would be using shared mmaps for their buffer cache.

I think it is safest to make a default policy of asking for IOs against private
cow-able mappings to be quiesced before fork, so there are no surprises
or reliance on COW details in the mm. Do you think?
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