linux-kernel - Re: [PATCH v1 10/15] mm/page-flags: reuse PG_slab as PG_anon

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Message-ID: <CAHk-=wjWx_bPBLB=qMMae8Sy3KrO+Kvaf4juPknO5HX-+Ot0XQ@mail.gmail.com>
Date:   Fri, 11 Mar 2022 11:22:37 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v1 10/15] mm/page-flags: reuse PG_slab as
 PG_anon_exclusive for PageAnon() pages

On Fri, Mar 11, 2022 at 10:46 AM David Hildenbrand <david@...hat.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