mm, hwpoison, hugetlb: introduce SUBPAGE_INDEX_HWPOISON to save raw error page

mm, hwpoison, hugetlb: introduce SUBPAGE_INDEX_HWPOISON to save raw error page

This patchset enables memory error handling on 1GB hugepage.

"Save raw error page" patch (1/4 of patchset [1]) is necessary, so it's
included in this series (the remaining part of hotplug related things are
still in progress).  Patch 2/5 solves issues in a corner case of hugepage
handling, which might not be the main target of this patchset, but
slightly related.  It was posted separately [2] but depends on 1/5, so I
group them together.

Patches 3/5 to 5/5 are the main part of this series and fix a small issue
about handling 1GB hugepage, which I hope will be workable.

[1]: https://lore.kernel.org/linux-mm/20220427042841.678351-1-naoya.horiguchi@linux.dev/T/#u

[2]: https://lore.kernel.org/linux-mm/20220511151955.3951352-1-naoya.horiguchi@linux.dev/T/

This patch (of 5):

When handling memory error on a hugetlb page, the error handler tries to
dissolve and turn it into 4kB pages.  If it's successfully dissolved,
PageHWPoison flag is moved to the raw error page, so that's all right.
However, dissolve sometimes fails, then the error page is left as
hwpoisoned hugepage.  It's useful if we can retry to dissolve it to save
healthy pages, but that's not possible now because the information about
where the raw error page is lost.

Use the private field of a tail page to keep that information.  The code
path of shrinking hugepage pool used this info to try delayed dissolve.
This only keeps one hwpoison page for now, which might be OK because it's
simple and multiple hwpoison pages in a hugepage can be rare.  But it can
be extended in the future.

Link: https://lkml.kernel.org/r/20220602050631.771414-1-naoya.horiguchi@linux.dev
Link: https://lkml.kernel.org/r/20220602050631.771414-2-naoya.horiguchi@linux.dev
Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>