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→Restoring
=== Restoring ===
=== Restoring ===
Restoring is pretty straightforward as during restore CRIU morphs itself into the target task. Two things worth mentioning.
Restoring is pretty straightforward as during restore CRIU morphs itself into the target task. Two things worth mentioning before diving into explanation of steps.
;[[COW]]
;[[COW]]
;[[Shared memory]]
;[[Shared memory]]
:Those areas are implemented in the kernel by supporting a pseudo file on a hidden tmpfs mount. So on restore we just determine who will create the shared are and who will attach to it (see the [[postulates]]). Then the creator <code>mmap</code>-s the region and the others open the /proc/pid/map_files/ link. However, on the recent kernels, we use the new <code>memfd</code> system call that does similar thing but works for user namespaces. Briefly -- creator creates the memfd, all the others get one via /proc/pid/fd link which is not that strict as compared to the map_files.
:Those areas are implemented in the kernel by supporting a pseudo file on a hidden tmpfs mount. So on restore we just determine who will create the shared are and who will attach to it (see the [[postulates]]). Then the creator <code>mmap</code>-s the region and the others open the /proc/pid/map_files/ link. However, on the recent kernels, we use the new <code>memfd</code> system call that does similar thing but works for user namespaces. Briefly -- creator creates the memfd, all the others get one via /proc/pid/fd link which is not that strict as compared to the map_files.
having said that, the restore of memory is done in the following steps
; Opening images and reading VMA-s in
: Here we open all the mm.img, read mappings in, resolve shared memory segments and check whether we need to special-care mapped files.
; Forking and pre-mapping
: At this stage each task pre-mmaps private anonymous areas and populates them with pages (from pagemap/pages images). Then task fork()-s the child which does the same. This is done like this to make COW-ed areas actually share the pages they should. On fork() the shared pages become such and currently that's the only way to make Linux kernel do this.
; Opening file mappings
: Soon after fork we check which VMA-s are MAP_FILE ones and request the [[files]] engine to open them.
; Opening shared mappings
: At almost the same place we create an FD for shared anonymous VMA-s.
; Diving into [[restorer context]]
: At this stage we strip off all the old CRIU mappings thus making the VM be ready for restored mappings.
; Restoring mappings in their places
: Anonymous private mappings are <code>mremap</code>-ed from the pre-mapped areas one-by-one, file mappings are created with <code>mmap</code> system call. Anonymous shared mappings are also just mmaped.
=== Non linear mappings ===
=== Non linear mappings ===