Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: 2021-04-20 07:40:21
Also in:
linux-mips, linux-mm, linuxppc-dev, lkml, netdev
Hi Willy, On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox [off-list ref] wrote:
Replacement patch to fix compiler warning.
From: "Matthew Wilcox (Oracle)" <willy@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: 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) <willy@infradead.org>Thanks for your patch!
quoted hunk ↗ jump to hunk
--- 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.
quoted hunk ↗ jump to hunk
}; 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
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