[PATCH v4 4/4] docs: mm: add enable_soft_offline sysctl

[Jiaqi Yan <jiaqiyan@google.com>]

Add the documentation for soft offline behaviors / costs, and what
the new enable_soft_offline sysctl is for.

Acked-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 Documentation/admin-guide/sysctl/vm.rst | 32 +++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index e86c968a7a0e..71463a7b3e2a 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -36,6 +36,7 @@ Currently, these files are in /proc/sys/vm:
 - dirtytime_expire_seconds
 - dirty_writeback_centisecs
 - drop_caches
+- enable_soft_offline
 - extfrag_threshold
 - highmem_is_dirtyable
 - hugetlb_shm_group
@@ -267,6 +268,37 @@ used::
 These are informational only.  They do not mean that anything is wrong
 with your system.  To disable them, echo 4 (bit 2) into drop_caches.
 
+enable_soft_offline
+===================
+Correctable memory errors are very common on servers. Soft-offline is kernel's
+solution for memory pages having (excessive) corrected memory errors.
+
+For different types of page, soft-offline has different behaviors / costs.
+- For a raw error page, soft-offline migrates the in-use page's content to
+  a new raw page.
+- For a page that is part of a transparent hugepage,  soft-offline splits the
+  transparent hugepage into raw pages, then migrates only the raw error page.
+  As a result, user is transparently backed by 1 less hugepage, impacting
+  memory access performance.
+- For a page that is part of a HugeTLB hugepage, soft-offline first migrates
+  the entire HugeTLB hugepage, during which a free hugepage will be consumed
+  as migration target.  Then the original hugepage is dissolved into raw
+  pages without compensation, reducing the capacity of the HugeTLB pool by 1.
+
+It is user's call to choose between reliability (staying away from fragile
+physical memory) vs performance / capacity implications in transparent and
+HugeTLB cases.
+
+For all architectures, enable_soft_offline controls whether to soft offline
+memory pages.  When setting to 1, kernel attempts to soft offline the pages
+whenever it thinks needed.  When setting to 0, kernel returns EOPNOTSUPP to
+the request to soft offline the pages.  Its default value is 1.
+
+It is worth mentioning that after setting enable_soft_offline to 0, the
+following requests to soft offline pages will not be performed:
+- Request to soft offline pages from RAS Correctable Errors Collector.
+- On ARM, the request to soft offline pages from GHES driver.
+- On PARISC, the request to soft offline pages from Page Deallocation Table.
 
 extfrag_threshold
 =================
-- 
2.45.2.741.gdbec12cfda-goog