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Tue, 20 Apr 2021 00:40:06 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=H6eQzhTjkJ0rVc7HEe9OWdxxcp9k4JKL+wOyVTTn6UM=; b=B8pmVOhVrvB6tyPsFP3fkWK+HDEsJ2tu+2JQznb7iC9q05Vt4Wk7RUoFM5jVWUKuXV S62bK007cQzFewgMXARfjOBjpGZ0p25DwqGMYzuscRE6Ons0DWW74g3f7oL1fCe7CFxU wpk9RBvzJzJepmiOSndRIHKlaAobQCy59yZFtMbvqH1FXU4fgWDBIspLsj950w9EYeqD xffkhgzSBCF6smnej0VKWD1STBzG8ErkhCyLUDNkVWHjFneGTEg9btoQAZcQgcjPaM1W feVCaLNso63leU+stgq5/LZShn3dQ6seqC/xLfPY0cycOkXva4BzTS2yOHPKInp8Hibq zSOw== X-Gm-Message-State: AOAM531ItMwCKJyFODZP4qr/zhRExI2+kqZelNIBhRIw/npSFFVYKTQP G3EYzm6wnCHBJOwIVQFNTYMPcHKhVEG6LTxs7DQ= X-Google-Smtp-Source: ABdhPJyQrVlZDIuzz0IXIFr61zWg3jELX2WXl+k/TdsCheRdiMxsjqJCL3H7m7n2s4eAxB2N41JJBdYT1MQaqIXF0Cc= X-Received: by 2002:a1f:2504:: with SMTP id l4mr15813452vkl.5.1618904406471; Tue, 20 Apr 2021 00:40:06 -0700 (PDT) MIME-Version: 1.0 References: <20210416230724.2519198-1-willy@infradead.org> <20210416230724.2519198-2-willy@infradead.org> <20210417024522.GP2531743@casper.infradead.org> In-Reply-To: <20210417024522.GP2531743@casper.infradead.org> From: Geert Uytterhoeven Date: Tue, 20 Apr 2021 09:39:54 +0200 Message-ID: Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems To: Matthew Wilcox Cc: Jesper Dangaard Brouer , Linux Kernel Mailing List , Linux MM , netdev , linuxppc-dev , Linux ARM , "open list:BROADCOM NVRAM DRIVER" , Ilias Apalodimas , mcroce@linux.microsoft.com, Grygorii Strashko , Arnd Bergmann , Christoph Hellwig , arcml , Michal Hocko , Mel Gorman Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E91DEE000105 X-Stat-Signature: nr8jgcrfx5kjffrpc3x7f1s118n7o3y1 Received-SPF: none (gmail.com>: No applicable sender policy available) receiver=imf30; identity=mailfrom; envelope-from=""; helo=mail-vk1-f172.google.com; client-ip=209.85.221.172 X-HE-DKIM-Result: none/none X-HE-Tag: 1618904392-990614 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hi Willy, On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox wrote: > Replacement patch to fix compiler warning. > > From: "Matthew Wilcox (Oracle)" > Date: Fri, 16 Apr 2021 16:34:55 -0400 > Subject: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems > To: brouer@redhat.com > Cc: linux-kernel@vger.kernel.org, > linux-mm@kvack.org, > netdev@vger.kernel.org, > linuxppc-dev@lists.ozlabs.org, > linux-arm-kernel@lists.infradead.org, > linux-mips@vger.kernel.org, > ilias.apalodimas@linaro.org, > mcroce@linux.microsoft.com, > grygorii.strashko@ti.com, > arnd@kernel.org, > hch@lst.de, > linux-snps-arc@lists.infradead.org, > mhocko@kernel.org, > mgorman@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) 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 -- geert@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