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Date:   Thu, 1 Apr 2021 13:52:01 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Christoph Hellwig <hch@infradead.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-cachefs@redhat.com, linux-afs@lists.infradead.org
Subject: Re: [PATCH v6 00/27] Memory Folios
Message-ID: <20210401125201.GD351017@casper.infradead.org>
References: <20210331184728.1188084-1-willy@infradead.org>
 <20210401070537.GB1363493@infradead.org>
 <20210401112656.GA351017@casper.infradead.org>
 <20210401122803.GB2710221@ziepe.ca>
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On Thu, Apr 01, 2021 at 09:28:03AM -0300, Jason Gunthorpe wrote:
> On Thu, Apr 01, 2021 at 12:26:56PM +0100, Matthew Wilcox wrote:
> > On Thu, Apr 01, 2021 at 08:05:37AM +0100, Christoph Hellwig wrote:
> > > On Wed, Mar 31, 2021 at 07:47:01PM +0100, Matthew Wilcox (Oracle) wrote:
> > > >  - Mirror members of struct page (for pagecache / anon) into struct folio,
> > > >    so (eg) you can use folio->mapping instead of folio->page.mapping
> > > 
> > > Eww, why?
> > 
> > So that eventually we can rename page->mapping to page->_mapping and
> > prevent the bugs from people doing page->mapping on a tail page.  eg
> > https://lore.kernel.org/linux-mm/alpine.LSU.2.11.2103102214170.7159@eggly.anvils/
> 
> Is that gcc structure layout randomization stuff going to be a problem
> here?
> 
> Add some 
>   static_assert(offsetof(struct folio,..) == offsetof(struct page,..))
> 
> tests to force it?

You sound like the kind of person who hasn't read patch 1.

diff --git a/mm/util.c b/mm/util.c
index 0b6dd9d81da7..521a772f06eb 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -686,6 +686,25 @@ struct anon_vma *page_anon_vma(struct page *page)
 	return __page_rmapping(page);
 }
 
+static inline void folio_build_bug(void)
+{
+#define FOLIO_MATCH(pg, fl)						\
+BUILD_BUG_ON(offsetof(struct page, pg) != offsetof(struct folio, fl));
+
+	FOLIO_MATCH(flags, flags);
+	FOLIO_MATCH(lru, lru);
+	FOLIO_MATCH(mapping, mapping);
+	FOLIO_MATCH(index, index);
+	FOLIO_MATCH(private, private);
+	FOLIO_MATCH(_mapcount, _mapcount);
+	FOLIO_MATCH(_refcount, _refcount);
+#ifdef CONFIG_MEMCG
+	FOLIO_MATCH(memcg_data, memcg_data);
+#endif
+#undef FOLIO_MATCH
+	BUILD_BUG_ON(sizeof(struct page) != sizeof(struct folio));
+}
+
 struct address_space *page_mapping(struct page *page)
 {
 	struct address_space *mapping;