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.
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.
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
