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

[Nick Piggin <npiggin@gmail.com>]

On 2 May 2012 13:04, Hugh Dickins <hughd@google.com> wrote:
> On Wed, 2 May 2012, Nick Piggin wrote:
>> On 2 May 2012 03:56, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
>> >
>> > In the light of all of the comments, can someone revise the man-pages
>> > patch that Jan sent?
>>
>> This does not quite describe the entire situation, but something understandable
>> to developers:
>>
>> O_DIRECT IOs should never be run concurrently with fork(2) system call,
>> when the memory buffer is anonymous memory, or comes from mmap(2)
>> with MAP_PRIVATE.
>>
>> Any such IOs, whether submitted with asynchronous IO interface or from
>> another thread in the process, should be quiesced before fork(2) is called.
>> Failure to do so can result in data corruption and undefined behavior in
>> parent and child processes.
>>
>> This restriction does not apply when the memory buffer for the O_DIRECT
>> IOs comes from mmap(2) with MAP_SHARED or from shmat(2).
>
> Nor does this restriction apply when the memory buffer has been advised
> as MADV_DONTFORK with madvise(2), ensuring that it will not be available
> to the child after fork(2).

Yes of course, I forgot that was exported too.

>
>>
>>
>>
>> Is that on the right track? I feel it might be necessary to describe this
>> allowance for MAP_SHARED, because some databases may be doing
>> such things, and anyway it gives apps a potential way to make this work
>> if concurrent fork + DIO is very important.
>
> Looks good, but we do need a reference to MADV_DONTFORK, perhaps as above.

Yep, thanks Hugh.

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