Hello, John and Peter On Mon, Oct 07, 2013 at 04:14:21PM -0700, John Stultz wrote: > On 10/07/2013 03:56 PM, H. Peter Anvin wrote: > > On 10/02/2013 05:51 PM, John Stultz wrote: > >> From: Minchan Kim <minchan@xxxxxxxxxx> > >> > >> This patch adds new system call sys_vrange. > >> > >> NAME > >> vrange - Mark or unmark range of memory as volatile > >> > > vrange() is about as nondescriptive as one can get -- there is exactly > > one letter that has any connection with that this does. > > > Hrm. Any suggestions? Would volatile_range() be better? > > > > > >> SYNOPSIS > >> int vrange(unsigned_long start, size_t length, int mode, > >> int *purged); > >> > >> DESCRIPTION > >> Applications can use vrange(2) to advise the kernel how it should > >> handle paging I/O in this VM area. The idea is to help the kernel > >> discard pages of vrange instead of reclaiming when memory pressure > >> happens. It means kernel doesn't discard any pages of vrange if > >> there is no memory pressure. > >> > >> mode: > >> VRANGE_VOLATILE > >> hint to kernel so VM can discard in vrange pages when > >> memory pressure happens. > >> VRANGE_NONVOLATILE > >> hint to kernel so VM doesn't discard vrange pages > >> any more. > >> > >> If user try to access purged memory without VRANGE_NOVOLATILE call, > >> he can encounter SIGBUS if the page was discarded by kernel. > >> > >> purged: Pointer to an integer which will return 1 if > >> mode == VRANGE_NONVOLATILE and any page in the affected range > >> was purged. If purged returns zero during a mode == > >> VRANGE_NONVOLATILE call, it means all of the pages in the range > >> are intact. > > I'm a bit confused about the "purged" > > > > From an earlier version of the patch: > > > >> - What's different with madvise(DONTNEED)? > >> > >> System call semantic > >> > >> DONTNEED makes sure user always can see zero-fill pages after > >> he calls madvise while vrange can see data or encounter SIGBUS. > > This difference doesn't seem to be a huge one. The other one seems to > > be the blocking status of MADV_DONTNEED, which perhaps may be better > > handled by adding an option (MADV_LAZY) perhaps? > > > > That way we would have lazy vs. immediate, and zero versus SIGBUS. > > And some sort of lazy-cancling call as well. > > > > > > I see from the change history of the patch that this was an madvise() at > > some point, but was changed into a separate system call at some point, > > does anyone remember why that was? A quick look through my LKML > > archives doesn't really make it clear. > > The reason we can't use madvise, is that to properly handle error cases > and report the pruge state, we need an extra argument. > > In much earlier versions, we just returned an error when setting > NONVOLATILE if the data was purged. However, since we have to possibly > do allocations when marking a range as non-volatile, we needed a way to > properly handle that allocation failing. We can't just return ENOMEM, as > we may have already marked purged memory as non-volatile. > > Thus, that's why with vrange, we return the number of bytes modified, > along with the purge state. That way, if an error does occur we can > return the purge state of the bytes successfully modified, and only > return an error if nothing was changed, much like when a write fails. As well, we might need addtional argument VRANGE_FULL/VRANGE_PARTIAL for vrange system call. I discussed it long time ago but omitted it for early easy review phase. It is requested by Mozilla fork and of course I think it makes sense to me. https://lkml.org/lkml/2013/3/22/20 In short, if you mark a range with VRANGE_FULL, kernel can discard all of pages within the range if memory is tight while kernel can discard part of pages in the vrange if you mark the range with VRANGE_PARTIAL. > > 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>