On Wed, Jun 19, 2013 at 01:31:28PM +0400, Pavel Emelyanov wrote: > >>> Maybe do you have a concern about live-lock? > >> > >> No, I worry about potential races with which we or application can skip > >> dirty page. Let me describe how CRIU uses existing soft-dirty implementation. > >> > >> 1. stop the task we want to work on > >> 2. read the /proc/pid/pagemap file to find out which pages to > >> read. Those with soft-dirty _cleared_ should be _skipped_ > >> 3. read task's memory at calculated bitmap > >> 4. reset soft dirty bits on task > >> 5. resume task execution > > > > Let me try to parse as my term. > > > > 1. admin does "echo 4 > /proc/<target>/clear_refs" > > 2. admin stop the target > > 3. admin reads the /proc/<target>/pagemap and make bitmap > > with only soft-dirty marked pages so we can avoid unnecessary > > migration > > 4. admin reads target's dirtied pages via bitmap from 3 > > 5. admin does "echo 4 > /proc/<target>/clear_refs" again to find > > future diry pages of the target. > > 6. admin resumes the target > > > > Right? > > Almost, the step #1 looks excessive. We shouldn't clear the soft dirty > _before_ stopping the target, otherwise we lose all the bits "collected" > before it. > > > If so, my interface is following as > > > > 1. admin does set_softdirty(target, 0, 0, &token); > > (set_softdirty clears all soft-dirty bit from target process's > > page table. > > 2. admin stop the target > > 3. admin reads the /proc/target/pagemap and make bitmap > > with only soft-dirty marked pages so we can avoid unnecessary > > migration. > > 4. admins does get_softdirty(target, 0, 0, token) to confirm > > someone else spoiled since 1 > > 4-1. If it is reports error, then admins discard the bitmap got > > from 3 and have to read all memory. > > 5. admin does set_softdirty(target, 0, 0, &token) again to find > > future dirty pages of the target > > 5. admin resumes the target. > > Same here -- if we skip step #1, then we can merge steps 4 and 5 into > one system call. Can we? > > >> > >> With the interface you propose the sequence presumably should look like > >> > >> 1. stop the task we want to work on > >> 2. call set_softdirty + get_softdirty to get the soft-dirty bitmap and > >> reset one. If it reports error, then the soft-dirty we did before is > >> spoiled and all memory should be read (iow -- bitmap should be filled > >> with 1-s) > >> 3. read task's memory at calculated bitmap > >> 4. resume task execution > > > >> > >> Am I right with this? If yes, why do we need two calls, wouldn't it be better > > > > I failed to parse your terms so I wrote scnario as my understanding > > so please see my above sequence and if you have a comment, please ask > > again. > > > >> to merge them into one? > > > > It's not hard part but I wanted to show my intention clearly. > > If we all agree on, let's think over interface again. > > For me the interface with a single syscall looks OK. If nobody else objects, > I think you can go on with the kernel patches :) Presumably you can even > use the criu project sources and tests to check how memory changes tracking > works with the new interface. Thanks for the good discussion! -- 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>