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Sat, 10 Apr 2021 06:22:08 +0000 (UTC) Received: from carbon (unknown [10.36.110.19]) by smtp.corp.redhat.com (Postfix) with ESMTP id DD3276F968; Sat, 10 Apr 2021 06:21:59 +0000 (UTC) Date: Sat, 10 Apr 2021 08:21:58 +0200 From: Jesper Dangaard Brouer To: Matthew Wilcox Subject: Re: Bogus struct page layout on 32-bit Message-ID: <20210410082158.79ad09a6@carbon> In-Reply-To: <20210410024313.GX2531743@casper.infradead.org> References: <20210409185105.188284-3-willy@infradead.org> <202104100656.N7EVvkNZ-lkp@intel.com> <20210410024313.GX2531743@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Ilias Apalodimas , kbuild-all@lists.01.org, kernel test robot , Ivan Khoronzhuk , clang-built-linux@googlegroups.com, brouer@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, "netdev@vger.kernel.org" , Paul Mackerras , linux-fsdevel@vger.kernel.org, Matteo Croce , linuxppc-dev@lists.ozlabs.org, "David S. Miller" , linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Sat, 10 Apr 2021 03:43:13 +0100 Matthew Wilcox wrote: > On Sat, Apr 10, 2021 at 06:45:35AM +0800, kernel test robot wrote: > > >> include/linux/mm_types.h:274:1: error: static_assert failed due to requirement '__builtin_offsetof(struct page, lru) == __builtin_offsetof(struct folio, lru)' "offsetof(struct page, lru) == offsetof(struct folio, lru)" > > FOLIO_MATCH(lru, lru); > > include/linux/mm_types.h:272:2: note: expanded from macro 'FOLIO_MATCH' > > static_assert(offsetof(struct page, pg) == offsetof(struct folio, fl)) > > Well, this is interesting. pahole reports: > > struct page { > long unsigned int flags; /* 0 4 */ > /* XXX 4 bytes hole, try to pack */ > union { > struct { > struct list_head lru; /* 8 8 */ > ... > struct folio { > union { > struct { > long unsigned int flags; /* 0 4 */ > struct list_head lru; /* 4 8 */ > > so this assert has absolutely done its job. > > But why has this assert triggered? Why is struct page layout not what > we thought it was? Turns out it's the dma_addr added in 2019 by commit > c25fff7171be ("mm: add dma_addr_t to struct page"). On this particular > config, it's 64-bit, and ppc32 requires alignment to 64-bit. So > the whole union gets moved out by 4 bytes. Argh, good that you are catching this! > Unfortunately, we can't just fix this by putting an 'unsigned long pad' > in front of it. It still aligns the entire union to 8 bytes, and then > it skips another 4 bytes after the pad. > > We can fix it like this ... > > +++ b/include/linux/mm_types.h > @@ -96,11 +96,12 @@ struct page { > unsigned long private; > }; > struct { /* page_pool used by netstack */ > + unsigned long _page_pool_pad; I'm fine with this pad. Matteo is currently proposing[1] to add a 32-bit value after @dma_addr, and he could use this area instead. [1] https://lore.kernel.org/netdev/20210409223801.104657-3-mcroce@linux.microsoft.com/ When adding/changing this, we need to make sure that it doesn't overlap member @index, because network stack use/check page_is_pfmemalloc(). As far as my calculations this is safe to add. I always try to keep an eye out for this, but I wonder if we could have a build check like yours. > /** > * @dma_addr: might require a 64-bit value even on > * 32-bit architectures. > */ > - dma_addr_t dma_addr; > + dma_addr_t dma_addr __packed; > }; > struct { /* slab, slob and slub */ > union { > > but I don't know if GCC is smart enough to realise that dma_addr is now > on an 8 byte boundary and it can use a normal instruction to access it, > or whether it'll do something daft like use byte loads to access it. > > We could also do: > > + dma_addr_t dma_addr __packed __aligned(sizeof(void *)); > > and I see pahole, at least sees this correctly: > > struct { > long unsigned int _page_pool_pad; /* 4 4 */ > dma_addr_t dma_addr __attribute__((__aligned__(4))); /* 8 8 */ > } __attribute__((__packed__)) __attribute__((__aligned__(4))); > > This presumably affects any 32-bit architecture with a 64-bit phys_addr_t > / dma_addr_t. Advice, please? I'm not sure that the 32-bit behavior is with 64-bit (dma) addrs. I don't have any 32-bit boards with 64-bit DMA. Cc. Ivan, wasn't your board (572x ?) 32-bit with driver 'cpsw' this case (where Ivan added XDP+page_pool) ? -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer