[PATCH v5 7/7] hugetlb_cgroup: Add hugetlb_cgroup reservation docs

[Mina Almasry <almasrymina@google.com>]

Add docs for how to use hugetlb_cgroup reservations, and their behavior.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Hillf Danton <hdanton@sina.com>

---
 .../admin-guide/cgroup-v1/hugetlb.rst         | 85 ++++++++++++++++---
 1 file changed, 74 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/hugetlb.rst b/Documentation/admin-guide/cgroup-v1/hugetlb.rst
index a3902aa253a9..70c10bd9a0b7 100644
--- a/Documentation/admin-guide/cgroup-v1/hugetlb.rst
+++ b/Documentation/admin-guide/cgroup-v1/hugetlb.rst
@@ -2,13 +2,6 @@
 HugeTLB Controller
 ==================

-The HugeTLB controller allows to limit the HugeTLB usage per control group and
-enforces the controller limit during page fault. Since HugeTLB doesn't
-support page reclaim, enforcing the limit at page fault time implies that,
-the application will get SIGBUS signal if it tries to access HugeTLB pages
-beyond its limit. This requires the application to know beforehand how much
-HugeTLB pages it would require for its use.
-
 HugeTLB controller can be created by first mounting the cgroup filesystem.

 # mount -t cgroup -o hugetlb none /sys/fs/cgroup
@@ -28,10 +21,14 @@ process (bash) into it.

 Brief summary of control files::

- hugetlb.<hugepagesize>.limit_in_bytes     # set/show limit of "hugepagesize" hugetlb usage
- hugetlb.<hugepagesize>.max_usage_in_bytes # show max "hugepagesize" hugetlb  usage recorded
- hugetlb.<hugepagesize>.usage_in_bytes     # show current usage for "hugepagesize" hugetlb
- hugetlb.<hugepagesize>.failcnt		   # show the number of allocation failure due to HugeTLB limit
+ hugetlb.<hugepagesize>.reservation_limit_in_bytes     # set/show limit of "hugepagesize" hugetlb reservations
+ hugetlb.<hugepagesize>.reservation_max_usage_in_bytes # show max "hugepagesize" hugetlb reservations recorded
+ hugetlb.<hugepagesize>.reservation_usage_in_bytes     # show current reservations for "hugepagesize" hugetlb
+ hugetlb.<hugepagesize>.reservation_failcnt            # show the number of allocation failure due to HugeTLB reservation limit
+ hugetlb.<hugepagesize>.limit_in_bytes                 # set/show limit of "hugepagesize" hugetlb faults
+ hugetlb.<hugepagesize>.max_usage_in_bytes             # show max "hugepagesize" hugetlb  usage recorded
+ hugetlb.<hugepagesize>.usage_in_bytes                 # show current usage for "hugepagesize" hugetlb
+ hugetlb.<hugepagesize>.failcnt                        # show the number of allocation failure due to HugeTLB usage limit

 For a system supporting three hugepage sizes (64k, 32M and 1G), the control
 files include::
@@ -40,11 +37,77 @@ files include::
   hugetlb.1GB.max_usage_in_bytes
   hugetlb.1GB.usage_in_bytes
   hugetlb.1GB.failcnt
+  hugetlb.1GB.reservation_limit_in_bytes
+  hugetlb.1GB.reservation_max_usage_in_bytes
+  hugetlb.1GB.reservation_usage_in_bytes
+  hugetlb.1GB.reservation_failcnt
   hugetlb.64KB.limit_in_bytes
   hugetlb.64KB.max_usage_in_bytes
   hugetlb.64KB.usage_in_bytes
   hugetlb.64KB.failcnt
+  hugetlb.64KB.reservation_limit_in_bytes
+  hugetlb.64KB.reservation_max_usage_in_bytes
+  hugetlb.64KB.reservation_usage_in_bytes
+  hugetlb.64KB.reservation_failcnt
   hugetlb.32MB.limit_in_bytes
   hugetlb.32MB.max_usage_in_bytes
   hugetlb.32MB.usage_in_bytes
   hugetlb.32MB.failcnt
+  hugetlb.32MB.reservation_limit_in_bytes
+  hugetlb.32MB.reservation_max_usage_in_bytes
+  hugetlb.32MB.reservation_usage_in_bytes
+  hugetlb.32MB.reservation_failcnt
+
+
+1. Reservation limits
+
+The HugeTLB controller allows to limit the HugeTLB reservations per control
+group and enforces the controller limit at reservation time. Reservation limits
+are superior to Page fault limits (see section 2), since Reservation limits are
+enforced at reservation time, and never causes the application to get SIGBUS
+signal. Instead, if the application is violating its limits, then it gets an
+error on reservation time, i.e. the mmap or shmget return an error.
+
+
+2. Page fault limits
+
+The HugeTLB controller allows to limit the HugeTLB usage (page fault) per
+control group and enforces the controller limit during page fault. Since HugeTLB
+doesn't support page reclaim, enforcing the limit at page fault time implies
+that, the application will get SIGBUS signal if it tries to access HugeTLB
+pages beyond its limit. This requires the application to know beforehand how
+much HugeTLB pages it would require for its use.
+
+
+3. Caveats with shared memory
+
+a. Charging and uncharging:
+
+For shared hugetlb memory, both hugetlb reservation and usage (page faults) are
+charged to the first task that causes the memory to be reserved or faulted,
+and all subsequent uses of this reserved or faulted memory is done without
+charging.
+
+Shared hugetlb memory is only uncharged when it is unreserved or deallocated.
+This is usually when the hugetlbfs file is deleted, and not when the task that
+caused the reservation or fault has exited.
+
+b. Interaction between reservation limit and fault limit.
+
+Generally, it's not recommended to set both of the reservation limit and fault
+limit in a cgroup. For private memory, the fault usage cannot exceed the
+reservation usage, so if you set both, one of those limits will be useless.
+Still, there is no kernel enforcement on setting both at the same time.
+
+For shared memory, a cgroup's fault usage may be greater than its reservation
+usage, so some care needs to be taken. Consider this example:
+
+- Task A reserves 4 pages in a shared hugetlbfs file. Cgroup A will get
+  4 reservations charged to it and no faults charged to it.
+- Task B reserves and faults the same 4 pages as Task A. Cgroup B will get no
+  reservation charge, but will get charged 4 faulted pages. If Cgroup B's limit
+  is less than 4, then Task B will get a SIGBUS.
+
+For the above scenario, it's not recommended for the userspace to set both
+reservation limits and fault limits, but it is still allowed to in case it sees
+some use for it, such as safe gradual transitioning from one to the other.
--
2.23.0.351.gc4317032e6-goog