On 9 May 2012 15:35, Michael Kerrisk (man-pages) <mtk.manpages@xxxxxxxxx> wrote: > On Wed, May 9, 2012 at 11:10 AM, Nick Piggin <npiggin@xxxxxxxxx> wrote: >> On 6 May 2012 01:29, KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> wrote: >>>> So, am I correct to assume that right text to add to the page is as below? >>>> >>>> Nick, can you clarify what you mean by "quiesced"? >>> >>> finished? >> >> Yes exactly. That might be a simpler word. Thanks! > > Thanks. > > But see below. I realize the text is still ambiguous. > >>>> [[ >>>> 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). >>>> ]] > > In the above, the status of a MAP_SHARED MAP_ANONYMOUS buffer is > unclear. The first paragraph implies that such a buffer is unsafe, > while the third paragraph implies that it *is* safe, thus > contradicting the first paragraph. Which is correct? Yes I see. It's because MAP_SHARED | MAP_ANONYMOUS isn't *really* anonymous from the virtual memory subsystem's point of view. But that just serves to confuse userspace I guess. Anything with MAP_SHARED, shmat, or MADV_DONTFORK is OK. Anything else (MAP_PRIVATE, brk, without MADV_DONTFORK) is dangerous. These type are used by standard heap allocators malloc, new, etc. -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
