On Wed, 05 Dec 2012 13:53:17 +0400 Pavel Emelyanov <xemul@xxxxxxxxxxxxx> wrote: > 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. How is all this going to work, btw? What is the interface to query page states and set them read-only? How will dirty pagecache and dirty swapcache be handled? And anonymous memory? -- 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>