On Thu, May 08, 2014 at 09:38:40AM -0700, John Stultz wrote: > 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? MADV_FREE is one-shot operation. I mean we should call it again to make them lazyfree while vrange could preserve volatility. Pz, think about thread-sanitizer usecase. They do mmap 70TB once start up and want to mark the range as volatile. If they uses MADV_FREE instead of volatile, they should mark 70TB as lazyfree periodically, which is terrible because MADV_FREE's cost is O(N). > > 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. I know. New vrange syscall which works with existing VMA instead of new vrange interval tree removed big concern from mm folks about duplicating of manage layer(ex, vm_area_struct and vrange inteval tree) and it removed my concern that mmap_sem write-side lock scalability for allocator usecase so we can make the implemenation simple and clear. I like it but zero-page VS SIGBUS is another issue we should make an agreement. > > > > 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. I already explained above why MADV_FREE is not enough. > > thanks > -john > > > > -- > 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> -- Kind regards, Minchan Kim -- 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>