Re: [PATCH v3 3/3] docs: mm: add enable_soft_offline sysctl

[Oscar Salvador <osalvador@suse.de>]

On Mon, Jun 17, 2024 at 05:05:45PM +0000, Jiaqi Yan wrote:
> Add the documentation for soft offline behaviors / costs, and what
> the new enable_soft_offline sysctl is for.
> 
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
> ---
>  Documentation/admin-guide/sysctl/vm.rst | 33 +++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index e86c968a7a0e..fc62fc272fc5 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 +268,38 @@ 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:
> +- If RAS Correctable Errors Collector is running, its request to soft offline
> +  pages will fail.
> +- On ARM, the request to soft offline pages from GHES driver will fail.
> +- On PARISC, the request to soft offline pages from Page Deallocation Table
> +  will fail.

I do not know about others but the 'fail' word feels wrong here.
I would reword that as "... the request to soft offline pages from
xxxx will not be performed".

 
Other than that:

Acked-by: Oscar Salvador <osalvador@suse.de>

-- 
Oscar Salvador
SUSE Labs