When handling shmem page fault the THP with corrupted subpage could be
PMD mapped if certain conditions are satisfied.  But kernel is supposed
to send SIGBUS when trying to map hwpoisoned page.

There are two paths which may do PMD map: fault around and regular
fault.

Before commit f9ce0be7 ("mm: Cleanup faultaround and finish_fault()
codepaths") the thing was even worse in fault around path.  The THP
could be PMD mapped as long as the VMA fits regardless what subpage is
accessed and corrupted.  After this commit as long as head page is not
corrupted the THP could be PMD mapped.

In the regular fault path the THP could be PMD mapped as long as the
corrupted page is not accessed and the VMA fits.

This loophole could be fixed by iterating every subpage to check if any
of them is hwpoisoned or not, but it is somewhat costly in page fault
path.

So introduce a new page flag called HasHWPoisoned on the first tail
page.  It indicates the THP has hwpoisoned subpage(s).  It is set if any
subpage of THP is found hwpoisoned by memory failure and after the
refcount is bumped successfully, then cleared when the THP is freed or
split.

The soft offline path doesn't need this since soft offline handler just
marks a subpage hwpoisoned when the subpage is migrated successfully.
But shmem THP didn't get split then migrated at all.

Link: https://lkml.kernel.org/r/20211020210755.23964-3-shy828301@gmail.com
Fixes: 800d8c63 ("shmem: add huge pages support")
Signed-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>