> On Nov 16, 2015, at 10:13, Minchan Kim <minchan@xxxxxxxxxx> wrote: > > 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. > i think use access bit is not easy to implement for ANON page in kernel. we are sure the Anon page is always PageDirty() if it is !PageSwapCache() , unless it is MADV_FREE page , but use access bit , how to distinguish Normal ANON page and MADV_FREE page? it can be implemented by Access bit , but not easy, need more code change . Thanks -- 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