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

[Jiaqi Yan <jiaqiyan@google.com>]

On Tue, Jun 18, 2024 at 10:20 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> 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".

Will reword in v4.

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

Thanks Oscar!

>
> --
> Oscar Salvador
> SUSE Labs