mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap

Race between process_mrelease and exit_mmap, where free_pgtables is
called while __oom_reap_task_mm is in progress, leads to kernel crash
during pte_offset_map_lock call.  oom-reaper avoids this race by setting
MMF_OOM_VICTIM flag and causing exit_mmap to take and release
mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock.

Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to
fix this race, however that would be considered a hack.  Fix this race
by elevating mm->mm_users and preventing exit_mmap from executing until
process_mrelease is finished.  Patch slightly refactors the code to
adapt for a possible mmget_not_zero failure.

This fix has considerable negative impact on process_mrelease
performance and will likely need later optimization.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call")
Signed-off-by: Suren Baghdasaryan <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Jan Engelhardt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions