Messages in this thread | | | Subject | Re: [PATCH 4/6] mm: Make compound_head const-preserving | From | Vlastimil Babka <> | Date | Fri, 23 Apr 2021 16:58:14 +0200 |
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On 4/17/21 1:15 AM, Matthew Wilcox (Oracle) wrote: > If you pass a const pointer to compound_head(), you get a const pointer > back; if you pass a mutable pointer, you get a mutable pointer back.
Hmm, nice trick.
> Also remove an unnecessary forward definition of struct page; we're about > to dereference page->compound_head, so it must already have been defined.
Right.
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> --- > include/linux/page-flags.h | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 04a34c08e0a6..d8e26243db25 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -177,17 +177,17 @@ enum pageflags { > > #ifndef __GENERATING_BOUNDS_H > > -struct page; /* forward declaration */ > - > -static inline struct page *compound_head(struct page *page) > +static inline unsigned long _compound_head(const struct page *page) > { > unsigned long head = READ_ONCE(page->compound_head); > > if (unlikely(head & 1)) > - return (struct page *) (head - 1); > - return page; > + return head - 1; > + return (unsigned long)page; > } > > +#define compound_head(page) ((typeof(page))_compound_head(page)) > + > static __always_inline int PageTail(struct page *page) > { > return READ_ONCE(page->compound_head) & 1; >
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