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

[Hugh Dickins]

On Wed, 2 May 2012, Nick Piggin wrote:
> On 2 May 2012 03:56, Michael Kerrisk (man-pages) <mtk.manpages@xxxxxxxxx> 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).

> 
> 
> 
> 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.

Hugh
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