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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 11 Mar 2022 11:22:37 -0800
Message-ID: <CAHk-=wjWx_bPBLB=qMMae8Sy3KrO+Kvaf4juPknO5HX-+Ot0XQ@mail.gmail.com>
Subject: Re: [PATCH v1 10/15] mm/page-flags: reuse PG_slab as
 PG_anon_exclusive for PageAnon() pages
To:     David Hildenbrand <david@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
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        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
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        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
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        Linux-MM <linux-mm@kvack.org>
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On Fri, Mar 11, 2022 at 10:46 AM David Hildenbrand <david@redhat.com> wrote:
>
> -       PG_has_hwpoisoned = PG_mappedtodisk,
> +       PG_has_hwpoisoned = PG_waiters,

That makes me too nervous for words. PG_waiters is very subtle.

Not only is it magical in bit location ways (and the special
clear_bit_unlock_is_negative_byte() macro that *literally* exists only
for unlocking a page), it just ends up having fairly subtle semantics
with intentionally racy clearing etc.

Mixing that up with any hwpoison bits - that aren't used by any normal
mortals and thus get very little coverage - just sounds horribly
horribly wrong.

              Linus