Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 17 Jul 2023 17:17:18 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Ryan Roberts <ryan.roberts@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>, Yang Shi <shy828301@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v1 1/3] mm: Allow deferred splitting of arbitrary large
 anon folios
Message-ID: <ZLVpjhAvk/nAquiY@casper.infradead.org>
References: <20230717143110.260162-1-ryan.roberts@arm.com>
 <20230717143110.260162-2-ryan.roberts@arm.com>
 <ZLVeeDmPDWupYzSH@casper.infradead.org>
 <283e4122-c23f-35a1-4782-fddde32f4ad4@arm.com>
 <c10ae780-c405-b2d2-ecc0-78ef35a892f3@redhat.com>
 <ZLVkUlQXmPH1BXEx@casper.infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZLVkUlQXmPH1BXEx@casper.infradead.org>
Precedence: bulk

On Mon, Jul 17, 2023 at 04:54:58PM +0100, Matthew Wilcox wrote:
> Those are page->flags and page->compound_head for the third page in
> the folio.  They don't really need a name; nothing refers to them,
> but it's important that space not be reused ;-)
> 
> This is slightly different from _flags_1; we do have some flags which
> reuse the bits (they're labelled as PF_SECOND).  Right now, it's only
> PG_has_hwpoisoned, but we used to have PG_double_map.  Others may arise.

Sorry, this was incomplete.  We do still have per-page flags!  HWPoison
is the obvious one, but PG_head is per-page (... think about it ...)
PG_anon_exclusive is actually per-page.

Most of the flags labelled as PF_ANY are mislabelled.  PG_private and
PG_private2 are never set/cleared/tested on tail pages.  PG_young and
PG_idle are only ever tested on the head page, but some code incorrectly
sets them on tail pages, where those bits are ignored.  I tried to fix
that a while ago, but the patch was overlooked and I couldn't be bothered
to try all that hard.  I have no clue about PG_vmemmap_self_hosted.
I think PG_isolated is probably never set on compound pages.
PG_owner_priv_1 is a disaster, as you might expect.