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Subject[RFC PATCH 0/2] tracing/user_events: Remote write ABI
As part of the discussions for user_events aligned with user space
tracers, it was determined that user programs should register a 32-bit
value to set or clear a bit when an event becomes enabled. Currently a
shared page is being used that requires mmap().

In this new model during the event registration from user programs 2 new
values are specified. The first is the address to update when the event
is either enabled or disabled. The second is the bit to set/clear to
reflect the event being enabled. This allows for a local 32-bit value in
user programs to support both kernel and user tracers. As an example,
setting bit 31 for kernel tracers when the event becomes enabled allows
for user tracers to use the other bits for ref counts or other flags.
The kernel side updates the bit atomically, user programs need to also
update these values atomically.

User provided addresses must be aligned on a 32-bit boundary, this
allows for single page checking and prevents odd behaviors such as a
32-bit value straddling 2 pages instead of a single page.

When page faults are encountered they are done asyncly via a workqueue.
If the page faults back in, the write update is attempted again. If the
page cannot fault-in, then we log and wait until the next time the event
is enabled/disabled. This is to prevent possible infinite loops resulting
from bad user processes unmapping or changing protection values after
registering the address.

User programs that wish to have the enable bit shared across forks
either need to use a MAP_SHARED allocated address or register a new
address and file descriptor. If MAP_SHARED cannot be used or new
registrations cannot be done, then it's allowable to use MAP_PRIVATE
as long as the forked children never update the page themselves. Once
the page has been updated, the page from the parent will be copied over
to the child. This new copy-on-write page will not receive updates from
the kernel until another registration has been performed with this new

Beau Belgrave (2):
tracing/user_events: Use remote writes for event enablement
tracing/user_events: Fixup enable faults asyncly

include/linux/user_events.h | 10 +-
kernel/trace/trace_events_user.c | 396 ++++++++++++++++++++-----------
2 files changed, 270 insertions(+), 136 deletions(-)

base-commit: 23758867219c8d84c8363316e6dd2f9fd7ae3049

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