On 2 May 2012 13:04, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > 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). 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. -- 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>