From 44195d1eba826a8af0afbcfa69ab4cc26f6ead7f Mon Sep 17 00:00:00 2001 From: Jiaqi Yan Date: Wed, 26 Jun 2024 05:08:18 +0000 Subject: [PATCH] docs: mm: add enable_soft_offline sysctl Add the documentation for soft offline behaviors / costs, and what the new enable_soft_offline sysctl is for. [jiaqiyan@google.com: fix kerneldoc warnings] Link: https://lkml.kernel.org/r/CACw3F52=GxTCDw-PqFh3-GDM-fo3GbhGdu0hedxYXOTT4TQSTg@mail.gmail.com [jiaqiyan@google.com: there are more blank lines needed] Link: https://lkml.kernel.org/r/CACw3F52_obAB742XeDRNun4BHBYtrxtbvp5NkUincXdaob0j1g@mail.gmail.com Link: https://lkml.kernel.org/r/20240626050818.2277273-5-jiaqiyan@google.com Signed-off-by: Jiaqi Yan Acked-by: Oscar Salvador Acked-by: Miaohe Lin Acked-by: David Rientjes Cc: Frank van der Linden Cc: Jane Chu Cc: Jonathan Corbet Cc: Lance Yang Cc: Muchun Song Cc: Naoya Horiguchi Cc: Randy Dunlap Cc: Shuah Khan Signed-off-by: Andrew Morton --- Documentation/admin-guide/sysctl/vm.rst | 38 +++++++++++++++++++++++++ 1 file changed, 38 insertions(+) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index e86c968a7a0e..f48eaa98d22d 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 +268,43 @@ 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 set to 1, kernel attempts to soft offline the pages +whenever it thinks needed. When set 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 =================