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pagemap: update BUDDY flag documentation

Since v4.6 the BUDDY flag is set for _all_ pages in the block
and no longer just for the first one.
This change was introduced by:
commit 832fc1de01ae ("/proc/kpageflags: return KPF_BUDDY for "tail" buddy pages")

Strictly speaking, this was an ABI change, but as nobody has noticed
since 2016, let's just update the documentation.

Link: https://lkml.kernel.org/r/20251122211920.3410371-1-richard@nod.at
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Richard Weinberger and committed by
Andrew Morton 84a8d467 b7dd80f8

+2 -1
+2 -1
Documentation/admin-guide/mm/pagemap.rst
··· 115 115 A free memory block managed by the buddy system allocator. 116 116 The buddy system organizes free memory in blocks of various orders. 117 117 An order N block has 2^N physically contiguous pages, with the BUDDY flag 118 - set for and _only_ for the first page. 118 + set for all pages. 119 + Before 4.6 only the first page of the block had the flag set. 119 120 15 - COMPOUND_HEAD 120 121 A compound page with order N consists of 2^N physically contiguous pages. 121 122 A compound page with order 2 takes the form of "HTTT", where H donates its