Hi Matthew, On Sat, Apr 17, 2021 at 03:45:22AM +0100, Matthew Wilcox 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> > --- > include/linux/mm_types.h | 4 ++-- > include/net/page_pool.h | 12 +++++++++++- > net/core/page_pool.c | 12 +++++++----- > 3 files changed, 20 insertions(+), 8 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 6613b26a8894..5aacc1c10a45 100644 > --- 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]; > }; > 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; > + 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;
The 'error' that was reported will never trigger right? I assume this was compiled with dma_addr_t as 32bits (so it triggered the compilation error), but the if check will never allow this codepath to run. If so can we add a comment explaining this, since none of us will remember why in 6 months from now? > } > > static inline bool is_page_pool_compiled_in(void) > diff --git a/net/core/page_pool.c b/net/core/page_pool.c > index ad8b0707af04..f014fd8c19a6 100644 > --- a/net/core/page_pool.c > +++ b/net/core/page_pool.c > @@ -174,8 +174,10 @@ static void page_pool_dma_sync_for_device(struct > page_pool *pool, > struct page *page, > unsigned int dma_sync_size) > { > + dma_addr_t dma_addr = page_pool_get_dma_addr(page); > + > dma_sync_size = min(dma_sync_size, pool->p.max_len); > - dma_sync_single_range_for_device(pool->p.dev, page->dma_addr, > + dma_sync_single_range_for_device(pool->p.dev, dma_addr, > pool->p.offset, dma_sync_size, > pool->p.dma_dir); > } > @@ -226,7 +228,7 @@ static struct page *__page_pool_alloc_pages_slow(struct > page_pool *pool, > put_page(page); > return NULL; > } > - page->dma_addr = dma; > + page_pool_set_dma_addr(page, dma); > > if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV) > page_pool_dma_sync_for_device(pool, page, pool->p.max_len); > @@ -294,13 +296,13 @@ void page_pool_release_page(struct page_pool *pool, > struct page *page) > */ > goto skip_dma_unmap; > > - dma = page->dma_addr; > + dma = page_pool_get_dma_addr(page); > > - /* When page is unmapped, it cannot be returned our pool */ > + /* When page is unmapped, it cannot be returned to our pool */ > dma_unmap_page_attrs(pool->p.dev, dma, > PAGE_SIZE << pool->p.order, pool->p.dma_dir, > DMA_ATTR_SKIP_CPU_SYNC); > - page->dma_addr = 0; > + page_pool_set_dma_addr(page, 0); > skip_dma_unmap: > /* This may be the last page returned, releasing the pool, so > * it is not safe to reference pool afterwards. > -- > 2.30.2 > Thanks /Ilias