Correctable memory errors are very common on servers with large amount of
memory, and are corrected by ECC.  Soft offline is kernel's additional
recovery handling for memory pages having (excessive) corrected memory
errors.  Impacted page is migrated to a healthy page if inuse; the
original page is discarded for any future use.

The actual policy on whether (and when) to soft offline should be
maintained by userspace, especially in case of an 1G HugeTLB page. 
Soft-offline dissolves the HugeTLB page, either in-use or free, into
chunks of 4K pages, reducing HugeTLB pool capacity by 1 hugepage.  If
userspace has not acknowledged such behavior, it may be surprised when
later failed to mmap hugepages due to lack of hugepages.  In case of a
transparent hugepage, it will be split into 4K pages as well; userspace
will stop enjoying the transparent performance.

In addition, discarding the entire 1G HugeTLB page only because of
corrected memory errors sounds very costly and kernel better not doing
under the hood.  But today there are at least 2 such cases doing so:
1. when GHES driver sees both GHES_SEV_CORRECTED and
   CPER_SEC_ERROR_THRESHOLD_EXCEEDED after parsing CPER.
2. RAS Correctable Errors Collector counts correctable errors per
   PFN and when the counter for a PFN reaches threshold
In both cases, userspace has no control of the soft offline performed
by kernel's memory failure recovery.

This commit gives userspace the control of softofflining any page: kernel
only soft offlines raw page / transparent hugepage / HugeTLB hugepage if
userspace has agreed to.  The interface to userspace is a new sysctl at
/proc/sys/vm/enable_soft_offline.  By default its value is set to 1 to
preserve existing behavior in kernel.  When set to 0, soft-offline (e.g. 
MADV_SOFT_OFFLINE) will fail with EOPNOTSUPP.

[jiaqiyan@google.com: v7]
  Link: https://lkml.kernel.org/r/20240628205958.2845610-3-jiaqiyan@google.com
Link: https://lkml.kernel.org/r/20240626050818.2277273-3-jiaqiyan@google.comSigned-off-by: Jiaqi Yan <jiaqiyan@google.com>
Acked-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>