On Fri, Nov 13, 2015 at 11:46:07AM -0800, Andy Lutomirski wrote: > On Fri, Nov 13, 2015 at 12:13 AM, Daniel Micay <danielmicay@xxxxxxxxx> wrote: > > On 13/11/15 02:03 AM, Minchan Kim wrote: > >> On Fri, Nov 13, 2015 at 01:45:52AM -0500, Daniel Micay wrote: > >>>> And now I am thinking if we use access bit, we could implment MADV_FREE_UNDO > >>>> easily when we need it. Maybe, that's what you want. Right? > >>> > >>> Yes, but why the access bit instead of the dirty bit for that? It could > >>> always be made more strict (i.e. access bit) in the future, while going > >>> the other way won't be possible. So I think the dirty bit is really the > >>> more conservative choice since if it turns out to be a mistake it can be > >>> fixed without a backwards incompatible change. > >> > >> Absolutely true. That's why I insist on dirty bit until now although > >> I didn't tell the reason. But I thought you wanted to change for using > >> access bit for the future, too. It seems MADV_FREE start to bloat > >> over and over again before knowing real problems and usecases. > >> It's almost same situation with volatile ranges so I really want to > >> stop at proper point which maintainer should decide, I hope. > >> Without it, we will make the feature a lot heavy by just brain storming > >> and then causes lots of churn in MM code without real bebenfit > >> It would be very painful for us. > > > > Well, I don't think you need more than a good API and an implementation > > with no known bugs, kernel security concerns or backwards compatibility > > issues. Configuration and API extensions are something for later (i.e. > > land a baseline, then submit stuff like sysctl tunables). Just my take > > on it though... > > > > As long as it's anonymous MAP_PRIVATE only, then the security aspects > should be okay. MADV_DONTNEED seems to work on pretty much any VMA, > and there's been long history of interesting bugs there. > > As for dirty vs accessed, an argument in favor of going straight to > accessed is that it means that users can write code like this without > worrying about whether they have a kernel that uses the dirty bit: > > x = mmap(...); > *x = 1; /* mark it present */ > > /* i'm done with it */ > *x = 1; > madvise(MADV_FREE, x, ...); > > wait a while; > > /* is it still there? */ > if (*x == 1) { > /* use whatever was cached there */ > } else { > /* reinitialize it */ > *x = 1; > } > > With the dirty bit, this will look like it works, but on occasion > users will lose the race where they probe *x to see if the data was > lost and then the data gets lost before the next write comes in. > > Sure, that load from *x could be changed to RMW or users could do a > dummy write (e.g. x[1] = 1; if (*x == 1) ...), but people might forget > to do that, and the caching implications are a little bit worse. I think your example is the case what people abuse MADV_FREE. What happens if the object(ie, x) spans multiple pages? User should know object's memory align and investigate all of pages which span the object. Hmm, I don't think it's good for API. > > Note that switching to RMW is really really dangerous. Doing: > > *x &= 1; > if (*x == 1) ...; > > is safe on x86 if the compiler generates: > > andl $1, (%[x]); > cmpl $1, (%[x]); > > but is unsafe if the compiler generates: > > movl (%[x]), %eax; > andl $1, %eax; > movl %eax, (%[x]); > cmpl $1, %eax; > > and even worse if the write is omitted when "provably" unnecessary. > > OTOH, if switching to the accessed bit is too much of a mess, then > using the dirty bit at first isn't so bad. Thanks! I want to use dirty bit first. About access bit, I don't want to say it to mess but I guess it would change a lot subtle thing for all architectures. Because we have used access bit as just *hint* for aging while dirty bit is really *critical marker* for system integrity. A example in x86, we don't keep accuracy of access bit for reducing TLB flush IPI. I don't know what technique other arches have used but they might have. Thanks. > > --Andy > > -- > Andy Lutomirski > AMA Capital Management, LLC -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>