On Thu, Jan 14, 2010 at 04:30:51PM +0900, KOSAKI Motohiro wrote: > > On Thu, Jan 14, 2010 at 04:02:42PM +0900, KOSAKI Motohiro wrote: > > > > On Thu, Jan 14, 2010 at 09:31:03AM +0900, KOSAKI Motohiro wrote: > > > > > > If application does mlockall(MCL_FUTURE) it is no longer possible to mmap > > > > > > file bigger than main memory or allocate big area of anonymous memory > > > > > > in a thread safe manner. Sometimes it is desirable to lock everything > > > > > > related to program execution into memory, but still be able to mmap > > > > > > big file or allocate huge amount of memory and allow OS to swap them on > > > > > > demand. MAP_UNLOCKED allows to do that. > > > > > > > > > > > > Signed-off-by: Gleb Natapov <gleb@xxxxxxxxxx> > > > > > > --- > > > > > > > > > > > > I get reports that people find this useful, so resending. > > > > > > > > > > This description is still wrong. It doesn't describe why this patch is useful. > > > > > > > > > I think the text above describes the feature it adds and its use > > > > case quite well. Can you elaborate what is missing in your opinion, > > > > or suggest alternative text please? > > > > > > My point is, introducing mmap new flags need strong and clearly use-case. > > > All patch should have good benefit/cost balance. the code can describe the cost, > > > but the benefit can be only explained by the patch description. > > > > > > I don't think this poor description explained bit benefit rather than cost. > > > you should explain why this patch is useful and not just pretty toy. > > > > > The benefit is that with this patch I can lock all of my application in > > memory except some very big memory areas. My use case is that I want to > > run virtual machine in such a way that everything related to machine > > emulator is locked into the memory, but guest address space can be > > swapped out at will. Guest address space is so huge that it is not > > possible to allocated it locked and then unlock. I was very surprised > > that current Linux API has no way to do it hence this patch. It may look > > like a pretty toy to you until some day you need this and has no way to > > do it. > > Hmm.. > Your answer didn't match I wanted. Then I don't get what you want. > few additional questions. > > - Why don't you change your application? It seems natural way than kernel change. There is no way to change my application and achieve what I've described in a multithreaded app. > - Why do you want your virtual machine have mlockall? AFAIK, current majority > virtual machine doesn't. It is absolutely irrelevant for that patch, but just because you ask I want to measure the cost of swapping out of a guest memory. > - If this feature added, average distro user can get any benefit? > ?! Is this some kind of new measure? There are plenty of much more invasive features that don't bring benefits to an average distro user. This feature can bring benefit to embedded/RT developers. > I mean, many application developrs want to add their specific feature > into kernel. but if we allow it unlimitedly, major syscall become > the trushbox of pretty toy feature soon. > And if application developer wants to extend kernel in a way that it will be possible to do something that was not possible before why is this a bad thing? I would agree with you if for my problem was userspace solution, but there is none. The mmap interface is asymmetric in regards to mlock currently. There is MAP_LOCKED, but no MAP_UNLOCKED. Why MAP_LOCKED is useful then? -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
