Messages in this thread | | | Subject | Re: [PATCH v2 1/2] mm/rmap: fix and simplify reusing mergeable anon_vma as parent when fork | From | Konstantin Khlebnikov <> | Date | Mon, 13 Jan 2020 14:07:18 +0300 |
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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.
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 >>>>> >>> >
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