1,072
edits
Changes
Jump to navigation
Jump to search
created (not finished)
Line 1:
Line 1:
+
This articles describes some intricacies of handling shared memory mappings, i.e. mappings that are shared between a few processes.
+
+== Checkpoint ==
+
+Every process has one or more mmaped files. Some mappings (for example, ones of shared libraries)
+are shared between a few processes. During the checkpointing, CRIU need to figure out
+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/$PID/map_files/</code>,
+noting the ''device'' and ''inode'' fields of the structure returned by fstatat(). This information
+is collected and sorted. Now, if any few processes have a mapping with same ''device'' and ''inode'',
+this mapping is a shared one and should be dumped as such.
+
+== Restore ==
+
+Upon restore, CRIU already knows which mappings are shared, and the trick is to restore them as such.
+For that, two different approaches are used, depending on the availability.
+
+The common part is, between the processes sharing a mapping, the one with the lowest PID
+among the group performs the actual <code>mmap()</code>, while all the others wait
+for the mapping to appear and, once it's available, use it.
+
+=== memfd ===
+
+Linux kernel v3.17 adds a [http://man7.org/linux/man-pages/man2/memfd_create.2.html memfd_create()]
+syscall. CRIU restore checks if it is available from the running kernel; it yes, it is used.
+
+FIXME how
+
+=== /proc/$PID/map_files/ ===
+
+This method is used if memfd is not available. The limitation is, /proc/$PID/map_files/ is not available
+for users inside user namespaces (due to security concerns), so it's not possible to use it if there
+are any user namespaces in the dump.
+
+FIXME how
+
+===
+
[[Category:Memory]]
[[Category:Memory]]
[[Category:Under the hood]]
[[Category:Under the hood]]
[[Category:Empty articles]]
[[Category:Empty articles]]