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Date:   Fri, 11 Mar 2022 20:36:37 +0100
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Subject: Re: [PATCH v1 10/15] mm/page-flags: reuse PG_slab as
 PG_anon_exclusive for PageAnon() pages
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To:     Linus Torvalds <torvalds@linux-foundation.org>
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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        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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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 11.03.22 20:22, Linus Torvalds wrote:
> 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.

Yes, but PG_has_hwpoisoned is located on the second subpage of a
compound page, not on the head page.

> 
> 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.

I used PG_error before, but felt like using a bit that is never ever
valid to be set/cleared/checked on a subpage would be even a better fit:

Note the:

  PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD) __CLEARPAGEFLAG(Waiters, waiters, PF_ONLY_HEAD)

whereby PF_ONLY_HEAD translates to:

"for compound page, callers only ever operate on the head page."


I can just switch to PG_error, but for the second subpage, PG_waiters
should be just fine (unless I am missing something important).

-- 
Thanks,

David / dhildenb