Re: [PATCH v5 4/4] docs: mm: add enable_soft_offline sysctl

[Randy Dunlap <rdunlap@infradead.org>]

Hi--

On 6/24/24 9:33 AM, Jiaqi Yan wrote:
> 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
> @@ -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

Use only one space after the comma ...................^

> +  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

                  When set to 1,

> +whenever it thinks needed.  When setting to 0, kernel returns EOPNOTSUPP to

                               When set to 0,

> +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
>  =================

-- 
~Randy