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== How it works now ==
== Problem ==
=== Restoring ===
Private anonymous mappings are tricky. They are declared to belong to a single process only and contain ''its'' data, but the Linux kernel optimizes the case when task calls fork() and creates a copy of itself. In this case all private anonymous mappings are "shared" between the parent and the child, but when either of them tries to modify the memory, the respective page is duplicated and the changes occur in the modifier's copy only.
When taking a dump of a process tree, it's totally correct to copy contents of all the anonymous private mappings independently and restore them in the same way -- just mmap and put the memory in there. But with this approach we effectively do the described memory duplication and thus increase memory usage by checkpointed and restore application.
To fix this, criu in version 0.3 and above does special tricks.
== How restore works to keep COW intact ==
We have different ideas how to restore {{Abbr|COW|Copy-on-write}}<ref>[[wikipedia:Copy-on-write]]</ref> memory. In a moment we even thought to use {{Abbr|KSM|Kernel Shared Memory}}<ref>[[wikipedia:Kernel SamePage Merging (KSM)]]</ref> for that. As result we found a good way for restoring COW memory (I guess). All {{Abbr|VMA|Virtual Memory Area}}s are restored in the same way as they were created. Here are two questions:
We have different ideas how to restore {{Abbr|COW|Copy-on-write}}<ref>[[wikipedia:Copy-on-write]]</ref> memory. In a moment we even thought to use {{Abbr|KSM|Kernel Shared Memory}}<ref>[[wikipedia:Kernel SamePage Merging (KSM)]]</ref> for that. As result we found a good way for restoring COW memory (I guess). All {{Abbr|VMA|Virtual Memory Area}}s are restored in the same way as they were created. Here are two questions:
# Which VMAs should be inherited?
# Which VMAs should be inherited?
# How to avoid intersections with crtools VMAs?
# How to avoid intersections with criu VMAs?
The first question is not resolved completely. Now a VMA is inherited if a parent has a VMA with the same start and end addresses. This covers 99% of cases, but it doesn't work if a VMA was moved.
The first question is not resolved completely. Now a VMA is inherited if a parent has a VMA with the same start and end addresses. This covers 99% of cases, but it doesn't work if a VMA was moved.
The second question is more interesting. Currently crtools reserves continuous space for all private VMAs, then restores all VMAs one by one in this space. Inherited VMAs are moved from a parent space. All VMAs are sorted by start addresses.
The second question is more interesting. Currently criu reserves continuous space for all private VMAs, then restores all VMAs one by one in this space. Inherited VMAs are moved from a parent space. All VMAs are sorted by start addresses.
[[File:cow.png]]
[[File:cow.png]]
In “restorer” all crtools’ VMAs are unmapped and private VMAs are space apart. The complexity of this algorithm is linear. Now it looks simple, but I spent a few hours to find it.
In “restorer” all criu’s VMAs are unmapped and private VMAs are space apart. The complexity of this algorithm is linear. Now it looks simple, but I spent a few hours to find it.
{{Out|“Complexity is easy; simplicity is difficult. -Georgy Shpagin”}}
{{Out|“Complexity is easy; simplicity is difficult. -Georgy Shpagin”}}
{{Like}}
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[[Category:Under the hood]]
[[Category:Memory]]