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Workaround a malfunction of soft-dirty bits clearing on Power9
(fix of commit c1bf1b973)

Issue #479 (bdwgc).

Make a page dirty twice in detect_soft_dirty_supported() clearing all
the soft-dirty bits in the middle.  If the 2nd write to a page is not
noticeable, then fallback to mprotect-based VDB.  This a workaround for
a bug observed, at least, in Fedora 36 kernel on Power9 CPU.

* os_dep.c [SOFT_VDB] (clear_soft_dirty_bits): New static function
(move part of code from GC_soft_read_dirty).
* os_dep.c [SOFT_VDB] (detect_soft_dirty_supported): Call
clear_soft_dirty_bits() and retry changing *vaddr to check whether the
latter is reflected in pagemap file; add comment.
* os_dep.c [SOFT_VDB] (GC_soft_read_dirty): Call clear_soft_dirty_bits.

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