On 05/07/2014 06:21 PM, Minchan Kim wrote: > Hey John, > > On Tue, Apr 29, 2014 at 02:21:21PM -0700, John Stultz wrote: >> This patch introduces MADV_VOLATILE/NONVOLATILE flags to madvise(), >> which allows for specifying ranges of memory as volatile, and able >> to be discarded by the system. >> >> This initial patch simply adds flag handling to madvise, and the >> vma handling, splitting and merging the vmas as needed, and marking >> them with VM_VOLATILE. >> >> No purging or discarding of volatile ranges is done at this point. >> >> This a simplified implementation which reuses some of the logic >> from Minchan's earlier efforts. So credit to Minchan for his work. > Remove purged argument is really good thing but I'm not sure merging > the feature into madvise syscall is good idea. > My concern is how we support user who don't want SIGBUS. > I believe we should support them because someuser(ex, sanitizer) really > want to avoid MADV_NONVOLATILE call right before overwriting their cache > (ex, If there was purged page for cyclic cache, user should call NONVOLATILE > right before overwriting to avoid SIGBUS).
So... Why not use MADV_FREE then for this case? Just to be clear, by moving back to madvise, I'm not trying to replace MADV_FREE. I think you're work there is still useful and splitting the semantics between the two is cleaner. > Moreover, this changes made unmarking cost O(N) so I'd like to avoid > NOVOLATILE syscall if possible. Well, I think that was made in v13, but yes. NONVOLATILE is currently an expensive operation in order to keep the semantics simpler, as requested by Johannes and Kosaki-san. > For me, SIGBUS is more special usecase for code pages but I believe > both are reasonable for each usecase so my preference is MADV_VOLATILE > is just zero-filled page and MADV_VOLATILE_SIGBUS, another new advise > if you really want to merge volatile range feature with madvise. This I disagree with. Even for non-code page cases, SIGBUS on volatile page access is important for normal users who might accidentally touch volatile data, so they know they are corrupting their data. I know Johannes suggested this is simply a use-after-free issue, but I really feel it results in having very strange semantics. And for those cases where there is a benefit to zero-fill, MADV_FREE seems more appropriate. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
