linux-kernel - Re: [PATCH] mm: Wire up tail page poisoning over ->mappings

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Message-ID: <ZN/tRLy0e+Iod9z3@casper.infradead.org>
Date:   Fri, 18 Aug 2023 23:14:28 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Yang Shi <shy828301@...il.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: Wire up tail page poisoning over ->mappings

On Tue, Aug 15, 2023 at 05:06:59PM -0400, Peter Xu wrote:
> I split this out from another rfc series.  Removed RFC tag because it
> wasn't for this patch but for the documentation updates.  I'll post the rfc
> part alone.  Comments welcomed, thanks.

I still hate it, as I explained here:

https://lore.kernel.org/linux-mm/ZNp7yUgUrIpILnXu@casper.infradead.org/

> > + * When the tail page's mapping field reused, it'll be exempted from
> > + * ->mapping poisoning and checks.  Also see the macro TAIL_MAPPING.
> > + */
> > +#define  TAIL_MAPPING_REUSED_MAX  (2)
> 
> More importantly, I think this is over-parametrisation.  If you start to
> use extra fields in struct folio, just change the code in page_alloc.c
> directly.