On 12/05/2012 04:38 AM, Matt Mackall wrote: > On Tue, 2012-12-04 at 16:24 -0800, Andrew Morton wrote: >> On Tue, 04 Dec 2012 18:17:08 -0600 >> Matt Mackall <mpm@xxxxxxxxxxx> wrote: >> >>> On Tue, 2012-12-04 at 15:21 -0800, Andrew Morton wrote: >>>> On Tue, 04 Dec 2012 09:15:10 +0400 >>>> Pavel Emelyanov <xemul@xxxxxxxxxxxxx> wrote: >>>> >>>>> >>>>>> Two alternatives come to mind: >>>>>> >>>>>> 1) Use /proc/pid/pagemap (Documentation/vm/pagemap.txt) in some >>>>>> fashion to determine which pages have been touched. >>> >>> [momentarily coming out of kernel retirement for old man rant] >>> >>> This is a popular interface anti-pattern. >>> >>> You shouldn't use an interface that gives you huge amount of STATE to >>> detect small amounts of CHANGE via manual differentiation. >> >> I'm not sure that's what checkpoint-restart will be doing. If we want >> to determine "which pages have been touched since the last checkpoint >> ten minutes ago" then that set of touched pages *is* state. And it's >> not "small"! > > Yeah, there is definitely a middle-ground here between "I want > high-frequency updates" and "I want to see the whole picture". > The filesystem analogy is backups: we don't have any good way to say > "find me all files changed since yesterday" short of "find all files". > The closest thing is explicit snapshotting. For what is required for checkpoint-restore is -- we want to query the kernel for "what pages has been written to since moment X". But this "moment X" is a little bit more tricky than just "mark all pages r/o". Consider we're doing this periodically. So when defining the moment X for the 2nd time we should query the "changed" state and remap the respective page r/o atomically. Full snapshot is actually not required, since we don't need to keep the old copy of a page that is written to. Just a sign, that this page was modified is OK. Thanks, Pavel -- 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>