On Mon, Jan 13, 2020 at 02:07:18PM +0300, Konstantin Khlebnikov wrote: >On 13/01/2020 03.33, Wei Yang wrote: >> On Sun, Jan 12, 2020 at 12:55:45PM +0300, Konstantin Khlebnikov wrote: >> > >> > >> > On 12/01/2020 01.38, Wei Yang wrote: >> > > On Fri, Jan 10, 2020 at 11:11:23AM +0300, Konstantin Khlebnikov wrote: >> > > [...] >> > > > > > > > >> > > > > > > > series of vma in parent with shared AV: >> > > > > > > > >> > > > > > > > SRC1 - AV0 >> > > > > > > > SRC2 - AV0 >> > > > > > > > SRC3 - AV0 >> > > > > > > > ... >> > > > > > > > SRCn - AV0 >> > > > > > > > >> > > > > > > > in child after fork >> > > > > > > > >> > > > > > > > DST1 - AV_OLD_1 (some old vma, picked by anon_vma_clone) plus DST1 is attached to same AVs as SRC1 >> > > > > > > > DST2 - AV_OLD_2 (other old vma) plus DST1 is attached to same AVs as SRC2 >> > > > > > > > DST2 - AV1 prev AV parent does not match AV0, no old vma found for reusing -> allocate new one (child of AV0) >> > > > > > > > DST3 - AV1 - DST2->AV->parent == SRC3->AV (AV0) -> share AV with prev >> > > > > > > > DST4 - AV1 - same thing >> > > > > > > > ... >> > > > > > > > DSTn - AV1 >> > > > > > > > >> > > >> > > To focus on the point, I rearranged the order a little. Suppose your following >> > > comments is explaining the above behavior. >> > > >> > > I've illustrated how two heuristics (reusing-old and sharing-prev) _could_ work together. >> > > But they both are optional. >> > > At cloning first vma SRC1 -> DST1 there is no prev to share anon vma, >> > > thus works common code which _could_ reuse old vma because it have to. >> > > If there is no old anon-vma which have to be reused then DST1 will allocate >> > > new anon-vma (AV1) and it will be used by DST2 and so on like on your picture. >> > > >> > > I agree with your 3rd paragraph, but confused with 2nd. >> > > >> > > At cloning first vma SRC1 -> DST1, there is no prev so anon_vma_clone() would >> > > pick up a reusable anon_vma. Here you named it AV_OLD_1. This looks good to >> > > me. But I am not sure why you would picked up AV_OLD_2 for DST2? In parent, >> > > SRC1 and SRC2 has the same anon_vma, AV0. So in child, DST1 and DST2 could >> > > also share the same anon_vma, AV_OLD_1. >> > > >> > > Sorry for my poor understanding, would you mind giving me more hint on this >> > > change? >> > >> > For DST2 heuristic "share-with-prev" will not work because if prev (DST1) >> > uses old AV (AV_OLD_1) and AV_OLD_1->parent isn't SRC2->AV (AV0). >> > So DST2 could only pick another old AV or allocate new. >> >> I know this behavior after your change, my question is why you want to do so. > >Because I want to keep both heuristics. >This seems most sane way of interaction between them. > I am not sure this is more sane. Still suggest to separate your idea into a new patch, so audience could analysis and notice the change clearly. Otherwise audience would be confused with this behavior. >Unfortunately even this patch is slightly broken. >Condition prev->anon_vma->parent == pvma->anon_vma doesn't guarantee that >prev vma has the same set of anon-vmas like current vma. >I.e. anon_vma_clone(vma, prev) might be not enough for keeping connectivity. >Building such case isn't trivial job but I see nothing that could prevent it. > >> >> > >> > My patch uses condition dst->prev->anon_vma->parent == src->anon_vma rather >> > than obvious src->prev->anon_vma == src->anon_vma because in this way it >> > eliminates all unwanted corner cases and explicitly verifies that we going to >> > share related anon-vma. >> > >> >> This do eliminates some corner case, but as you showed child and parent don't >> share the same AV topology. To keep the same AV topology is the purpose of my >> commit. >> >> I agree you found some bug that previous commit doesn't do it is expected. But >> since you change the design a little, I suggest you split this idea to a >> separate patch so that reviewer and audience in the future could understand >> your approach clearly. Otherwise audience would be confused and hard to track >> this change. >> >> For example, you describe the behavior after your change. The second vma would >> probably have a different AV from first vma. >> >> > Heuristic "reuse-old" uses fact that VMA links and AV parent chain are tracked >> > independently: when VMA reuses old AV it still links to all related AV even >> > if VMA->AV points into some old AV in the middle of inheritance chain. >> > >> > > >> > > > > > > >> > > > > > > Yes, your code works for DST3..DSTn. They will pick up AV1 since >> > > > > > > (DST2->AV->parent == SRC3->AV). >> > > > > > > >> > > > > > > My question is why DST1 and DST2 has different AV? The purpose of my patch >> > > > > > > tries to make child has the same topology and parent. So the ideal look of >> > > > > > > child is: >> > > > > > > >> > > > > > > DST1 - AV1 >> > > > > > > DST2 - AV1 >> > > > > > > DST2 - AV1 >> > > > > > > DST3 - AV1 >> > > > > > > DST4 - AV1 >> > > > > > > >> > > > > > > Would you mind putting more words on DST1 and DST2? I didn't fully understand >> > > > > > > the logic here. >> > > > > > > >> > > > > > > Thanks >> > > > > > > >> > > > > > >> > > > > > I think that the first version is doing the work as you expected, but been >> > > > > > revised in second version, to limits the number of users of reused old >> > > > > > anon(which is picked in anon_vma_clone() and keep the tree structure. >> > > > > > >> > > > > >> > > > > Any reason to reduce the reuse? Maybe I lost some point. >> > > > >> > > > > >> > > > > > > -- >> > > > > > > Wei Yang >> > > > > > > Help you, Help me >> > > > > >> > > >> -- Wei Yang Help you, Help me
