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

[Jeff Moyer]

KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> writes:

>> Hello,
>>
>> Thank you revisit this. But as far as my remember is correct, this issue is NOT
>> unaligned access issue. It's just get_user_pages(_fast) vs fork race issue. i.e.
>> DIRECT_IO w/ multi thread process should not use fork().
>
> The problem is, fork (and its COW logic) assume new access makes cow break,
> But page table protection can't detect a DMA write. Therefore DIO may override
> shared page data.

Hm, I've only seen this with misaligned or multiple sub-page-sized reads
in the same page.  AFAIR, aligned, page-sized I/O does not get split.
But, I could be wrong...

Cheers,
Jeff
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