On Wed 09-05-12 19:18:16, Michael Kerrisk (man-pages) wrote: > On Wed, May 9, 2012 at 7:01 PM, Nick Piggin <npiggin@xxxxxxxxx> wrote: > > 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. > > So, would the following text be okay: > > O_DIRECT I/Os should never be run concurrently with the fork(2) > system call, if the memory buffer is a private mapping (i.e., > any mapping created with the mmap(2) MAP_PRIVATE flag; this > includes memory allocated on the heap and statically allocated > buffers). Any such I/Os, whether submitted via an asynchronous > I/O interface or from another thread in the process, should be > completed 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 mem‐ > ory buffer for the O_DIRECT I/Os was created using shmat(2) or > mmap(2) with the MAP_SHARED flag. 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). This text looks OK, to me. Thanks for putting it together. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
