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? >> > > >> > > 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
