On Thu, May 08, 2014 at 05:24:41PM -0700, John Stultz wrote: > On 05/08/2014 05:07 PM, Minchan Kim wrote: > > On Thu, May 08, 2014 at 04:43:07PM -0700, John Stultz wrote: > >> On 05/08/2014 04:12 PM, Minchan Kim wrote: > >>> 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). > >> I still have had difficulty seeing the thread-sanitizer usage as a > >> generic enough model for other applications. I realize they want to > >> avoid marking and unmarking ranges (and they want that marking and > >> unmarking to be very cheap), but the zero-fill purged page (while still > >> preserving volatility) causes lots of *very* strange behavior: > > > > I don't think it's for only thread-sanitizer. > > Pz, think following usecase. > > > > Let's assume big volatile cache. > > If there is request for cache, it should find a object in a cache > > and if it found, it should call vrange(NOVOLATILE) right before > > passing it to the user and investigate it was purged or not. > > If it wasn't purged, cache manager could pass the object to the user. > > But it's circular cache so if there is no request from user, cache manager > > always overwrites objects so it could encounter SIGBUS easily > > so as current sematic, cache manager always should call vrange(NOVOLATILE) > > right before the overwriting. Otherwise, it should register SIGBUS handler > > to unmark volatile by page unit. SIGH. > > > > If we support zero-fill, cache manager could overwrite object without > > SIGBUS handling or vrange(NOVOLATILE) call right before overwriting. > > Just what we need is vrange(NOVOLATILE) call right before passing it > > to user. > > But that wouldn't work. If the page was purged half way through writing > it, we end up with a page of half zero data and half written data. What > would the page state be at that point? Purged? Not purged? You're right. Application might detect it with adding a sentinel in the header but I don't think it should be generic model with zero-fill semantic although some of application could do it. > > * If its not purged (since a write was done to the page after being > zero-filled), we will silently return to the user corrupted data. > > * If it is considered purged, how do we store that data? Since we > currently detect purged pages by checking if they are present when we > mark non-volatile. > > > This sort of zero-fill behavior on volatile pages only seems to make > sense if pages are written atomically. > > The SIGBUS handling solution you SIGH'ed at above actually seems > reasonable, because it would allow the page to be safely filled > atomically (marking it non-volatile, filling it and then re-marking it > volatile). Sure it would cost more, fast and wrong isn't really a valid > option. Got it. My scenario was totally broken so I don't insist on such model any more. First of all, let's go with SIGBUS model first if there is no strong requirement from user folks. Thanks for pointing out, John! > > > > > > >> * How do general applications know the difference between a purged page > >> and a valid empty page? > >> * When reading/writing a page, what happens if half-way the application > >> is preempted, and the page is purged? > >> * If a volatile page is purged, then zero-filled on a read or write, > >> what is its purged state when we're marking it non-volatile? > > Maybe above scenario goes your questions to VOID. > > I'm not sure I understand this. > > > > > >> These use cases don't seem completely baked, or maybe I've just not been > >> able to comprehend them yet. But I don't quite understand the desire to > >> prioritize this style of usage over other simpler and more well > >> established usage? > > I think it's one of typical usecase of vrange syscall. > > I apologize if I'm seeming stubborn, but I just can't see how it would > work sanely. > > 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>