Re: [PATCH RFC v2 1/3] mm: Add TAIL_MAPPING_REUSED_MAX

From: Peter Xu <peterx@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Mike Kravetz <mike.kravetz@oracle.com>,
	David Hildenbrand <david@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Yu Zhao <yuzhao@google.com>, Ryan Roberts <ryan.roberts@arm.com>,
	Yang Shi <shy828301@gmail.com>, Hugh Dickins <hughd@google.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH RFC v2 1/3] mm: Add TAIL_MAPPING_REUSED_MAX
Date: Mon, 14 Aug 2023 15:51:27 -0400	[thread overview]
Message-ID: <ZNqFv0AwkfDKExiw@x1n> (raw)
In-Reply-To: <ZNp7yUgUrIpILnXu@casper.infradead.org>

On Mon, Aug 14, 2023 at 08:08:57PM +0100, Matthew Wilcox wrote:
> On Mon, Aug 14, 2023 at 02:44:09PM -0400, Peter Xu wrote:
> > +/*
> > + * This macro defines the maximum tail pages (of a folio) that can have the
> > + * page->mapping field reused (offset 12 for 32bits, or 24 for 64bits).
> 
> No, don't say how many bytes into the structure something is.  It'll
> only get out of date.  If somebody needs to know, use pahole.

OK.

> 
> > + * 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.

One should at least also need to change __split_huge_page_tail() on the
BUG_ON() with the hard-coded "tail > 2"?

I wanted to link all these pieces together, and the use case is when anyone
would like to e.g. reuse tail page 3 of a folio, hence put the macro here.

-- 
Peter Xu