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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>