| Message ID | 20210416230724.2519198-3-willy@infradead.org |
|---|---|
| State | New |
| Headers | show |
| Series | Change struct page layout for page_pool | expand |
From: Matthew Wilcox (Oracle) <willy@infradead.org> > Sent: 17 April 2021 00:07 > > The net page_pool wants to use a magic value to identify page pool pages. > The best place to put it is in the first word where it can be clearly a > non-pointer value. That means shifting dma_addr up to alias with ->index, > which means we need to find another way to indicate page_is_pfmemalloc(). > Since page_pool doesn't want to set its magic value on pages which are > pfmemalloc, we can use bit 1 of compound_head to indicate that the page > came from the memory reserves. > ... > struct { /* page_pool used by netstack */ > - /** > - * @dma_addr: might require a 64-bit value on > - * 32-bit architectures. > - */ > + unsigned long pp_magic; > + unsigned long xmi; > + unsigned long _pp_mapping_pad; > unsigned long dma_addr[2]; > }; You've deleted the comment. I also think there should be a comment that dma_addr[0] must be aliased to ->index. (Or whatever all the exact requirements are.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Sat, Apr 17, 2021 at 09:13:45PM +0000, David Laight wrote: > > struct { /* page_pool used by netstack */ > > - /** > > - * @dma_addr: might require a 64-bit value on > > - * 32-bit architectures. > > - */ > > + unsigned long pp_magic; > > + unsigned long xmi; > > + unsigned long _pp_mapping_pad; > > unsigned long dma_addr[2]; > > }; > > You've deleted the comment. Yes. It no longer added any value. You can see dma_addr now occupies two words. > I also think there should be a comment that dma_addr[0] > must be aliased to ->index. That's not a requirement. Moving the pfmemalloc indicator is a requirement so that we _can_ use index, but there's no requirement about how index is used.
diff --git a/include/linux/mm.h b/include/linux/mm.h index 8ba434287387..44eab3f6d5ae 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1629,10 +1629,12 @@ struct address_space *page_mapping_file(struct page *page); static inline bool page_is_pfmemalloc(const struct page *page) { /* - * Page index cannot be this large so this must be - * a pfmemalloc page. + * This is not a tail page; compound_head of a head page is unused + * at return from the page allocator, and will be overwritten + * by callers who do not care whether the page came from the + * reserves. */ - return page->index == -1UL; + return page->compound_head & 2; } /* @@ -1641,12 +1643,12 @@ static inline bool page_is_pfmemalloc(const struct page *page) */ static inline void set_page_pfmemalloc(struct page *page) { - page->index = -1UL; + page->compound_head = 2; } static inline void clear_page_pfmemalloc(struct page *page) { - page->index = 0; + page->compound_head = 0; } /* diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 5aacc1c10a45..39f7163dcace 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -96,10 +96,9 @@ struct page { unsigned long private; }; struct { /* page_pool used by netstack */ - /** - * @dma_addr: might require a 64-bit value on - * 32-bit architectures. - */ + unsigned long pp_magic; + unsigned long xmi; + unsigned long _pp_mapping_pad; unsigned long dma_addr[2]; }; struct { /* slab, slob and slub */
The net page_pool wants to use a magic value to identify page pool pages. The best place to put it is in the first word where it can be clearly a non-pointer value. That means shifting dma_addr up to alias with ->index, which means we need to find another way to indicate page_is_pfmemalloc(). Since page_pool doesn't want to set its magic value on pages which are pfmemalloc, we can use bit 1 of compound_head to indicate that the page came from the memory reserves. Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> --- include/linux/mm.h | 12 +++++++----- include/linux/mm_types.h | 7 +++---- 2 files changed, 10 insertions(+), 9 deletions(-)