Hi Willy, On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox <wi...@infradead.org> wrote: > Replacement patch to fix compiler warning. > > From: "Matthew Wilcox (Oracle)" <wi...@infradead.org> > Date: Fri, 16 Apr 2021 16:34:55 -0400 > Subject: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems > To: bro...@redhat.com > Cc: linux-ker...@vger.kernel.org, > linux...@kvack.org, > net...@vger.kernel.org, > linuxppc-dev@lists.ozlabs.org, > linux-arm-ker...@lists.infradead.org, > linux-m...@vger.kernel.org, > ilias.apalodi...@linaro.org, > mcr...@linux.microsoft.com, > grygorii.stras...@ti.com, > a...@kernel.org, > h...@lst.de, > linux-snps-...@lists.infradead.org, > mho...@kernel.org, > mgor...@suse.de > > 32-bit architectures which expect 8-byte alignment for 8-byte integers > and need 64-bit DMA addresses (arc, arm, mips, ppc) had their struct > page inadvertently expanded in 2019. When the dma_addr_t was added, > it forced the alignment of the union to 8 bytes, which inserted a 4 byte > gap between 'flags' and the union. > > Fix this by storing the dma_addr_t in one or two adjacent unsigned longs. > This restores the alignment to that of an unsigned long, and also fixes a > potential problem where (on a big endian platform), the bit used to denote > PageTail could inadvertently get set, and a racing get_user_pages_fast() > could dereference a bogus compound_head(). > > Fixes: c25fff7171be ("mm: add dma_addr_t to struct page") > Signed-off-by: Matthew Wilcox (Oracle) <wi...@infradead.org>
Thanks for your patch! > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -97,10 +97,10 @@ struct page { > }; > struct { /* page_pool used by netstack */ > /** > - * @dma_addr: might require a 64-bit value even on > + * @dma_addr: might require a 64-bit value on > * 32-bit architectures. > */ > - dma_addr_t dma_addr; > + unsigned long dma_addr[2]; So we get two 64-bit words on 64-bit platforms, while only one is needed? Would unsigned long _dma_addr[sizeof(dma_addr_t) / sizeof(unsigned long)]; work? Or will the compiler become too overzealous, and warn about the use of ...[1] below, even when unreachable? I wouldn't mind an #ifdef instead of an if () in the code below, though. > }; > struct { /* slab, slob and slub */ > union { > diff --git a/include/net/page_pool.h b/include/net/page_pool.h > index b5b195305346..ad6154dc206c 100644 > --- a/include/net/page_pool.h > +++ b/include/net/page_pool.h > @@ -198,7 +198,17 @@ static inline void page_pool_recycle_direct(struct > page_pool *pool, > > static inline dma_addr_t page_pool_get_dma_addr(struct page *page) > { > - return page->dma_addr; > + dma_addr_t ret = page->dma_addr[0]; > + if (sizeof(dma_addr_t) > sizeof(unsigned long)) > + ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16; We don't seem to have a handy macro for a 32-bit left shift yet... But you can also avoid the warning using ret |= (u64)page->dma_addr[1] << 32; > + return ret; > +} > + > +static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr) > +{ > + page->dma_addr[0] = addr; > + if (sizeof(dma_addr_t) > sizeof(unsigned long)) > + page->dma_addr[1] = addr >> 16 >> 16; ... but we do have upper_32_bits() for a 32-bit right shift. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds