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

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

Hello,

>> 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?
>    Yes, I agree that (and MADV_DONTFORK) is probably the best thing to have
> in documentation. Otherwise it's a bit too hairy...

I neglected this issue for years because Linus asked who need this and
I couldn't
find real world usecase.

Ah, no, not exactly correct. Fujitsu proprietary database had such
usecase. But they
quickly fixed it. Then I couldn't find alternative usecase.

I'm not sure why you say "hairy". Do you mean you have any use case of this?

Thank you.