On 2 May 2012 13:04, Hugh Dickins [off-list ref] wrote:

On Wed, 2 May 2012, Nick Piggin wrote:

quoted

On 2 May 2012 03:56, Michael Kerrisk (man-pages) [off-list ref] wrote:

quoted

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.

quoted

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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