PF_NOFS is a per-process debug flag which is set in current->flags to
detect when a process is performing an unsafe allocation. All tasks
with PF_NOFS set must strictly use KM_PUSHPAGE for allocations because
if they enter direct reclaim and initiate I/O they may deadlock.
When debugging is disabled, any incorrect usage will be detected and
a call stack with a warning will be printed to the console. The flags
will then be automatically corrected to allow for safe execution. If
debugging is enabled this will be treated as a fatal condition.
To avoid any risk of conflicting with the existing PF_ flags. The
PF_NOFS bit shadows the rarely used PF_MUTEX_TESTER bit. Only when
CONFIG_RT_MUTEX_TESTER is not set, and we know this bit is unused,
will the PF_NOFS bit be valid. Happily, most existing distributions
ship a kernel with CONFIG_RT_MUTEX_TESTER disabled.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Brian Behlendorf
eb0f407a2b