mm: shmem: allow preserving file over FDBOX + KHO
For applications with a large amount of memory that takes time to
rebuild, reboots to consume kernel upgrades can be very expensive. FDBox
allows preserving file descriptors over kexec using KHO. Combining that
with memfd gives those applications reboot-persistent memory that they
can use to quickly save and reconstruct that state.
Since memfd is backed either by hugetlbfs or shmem, use shmem as the
first backend for memfd that is FDBOX + KHO capable.
To preserve the file's contents during KHO activation, the file's page
cache must be walked and all entries removed, and their indices stored.
Use the newly introduced shmem_undo_range_ops to achieve this. Walk each
entry and before truncating it, take a refcount on the folio so it does
not get freed, and store its physical address and index in the kho_mem
and indices arrays.
Swap pages, partial folios, and huge folios are not supported yet.
Encountering those results in an error.
On the restore side, an empty file is created and then the mems array
walked to insert the pages into the page cache. The logic in
shmem_alloc_and_add_folio() is roughly followed.
Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
2 files changed
