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This articles describes some intricacies of handling shared memory mappings, i.e. mappings that are shared between a few processes.

This articles describes some intricacies of handling shared memory mappings,

i.e. anonymous (not file-based) mappings that are shared between a few processes.

== Checkpoint ==

== Checkpoint ==

Every process has one or more mmaped files. Some mappings (for example, ones of shared libraries)

Every process has one or more mmaped files. Some mappings can be shared between a few processes.

are shared between a few processes. During the checkpointing, CRIU need to figure out

During the checkpointing, CRIU need to figure out all the shared mappings in order to dump them as such.

all the mappings that are shared in order to dump them as such.

It does so by performing <code>fstatat()</code> for each entry in <code>/proc/$PID/map_files/</code>,

It does so by performing <code>fstatat()</code> for each entry in <code>/proc/$PID/map_files/</code>,

== Restore ==

== Restore ==

During the restore, CRIU already knows which mappings are shared, so they need to be restored as shared.

During the restore, CRIU already knows which mappings are shared, so they need to be

restored as such. Here is how it is done.

 

Anonymous memory mappings, though, need some work to be restored as such. Here is how it is done.

Among all the processes sharing a mapping, the one with the lowest PID among the group

Among all the processes sharing a mapping, the one with the lowest PID among the group