From: Arnd Bergmann <arnd@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: kernel test robot <lkp@intel.com>, Linux-MM <linux-mm@kvack.org>,
kbuild-all@lists.01.org,
clang-built-linux <clang-built-linux@googlegroups.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Jesper Dangaard Brouer <brouer@redhat.com>,
"David S. Miller" <davem@davemloft.net>
Subject: Re: Bogus struct page layout on 32-bit
Date: Sat, 10 Apr 2021 21:10:47 +0200 [thread overview]
Message-ID: <CAK8P3a3uEGaEN-p06vFP+jwbFt3P=Bx4=aRN+kUyB4PcFPxLRg@mail.gmail.com> (raw)
In-Reply-To: <20210410024313.GX2531743@casper.infradead.org>
On Sat, Apr 10, 2021 at 4:44 AM Matthew Wilcox <willy@infradead.org> wrote:
> + 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've tried out what gcc would make of this: https://godbolt.org/z/aTEbxxbG3
struct page {
short a;
struct {
short b;
long long c __attribute__((packed, aligned(2)));
} __attribute__((packed));
} __attribute__((aligned(8)));
In this structure, 'c' is clearly aligned to eight bytes, and gcc does
realize that
it is safe to use the 'ldrd' instruction for 32-bit arm, which is forbidden on
struct members with less than 4 byte alignment. However, it also complains
that passing a pointer to 'c' into a function that expects a 'long long' is not
allowed because alignof(c) is only '2' here.
(I used 'short' here because I having a 64-bit member misaligned by four
bytes wouldn't make a difference to the instructions on Arm, or any other
32-bit architecture I can think of, regardless of the ABI requirements).
Arnd
next prev parent reply other threads:[~2021-04-10 19:11 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-09 18:50 [PATCH v7 00/28] Memory Folios Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 01/28] mm: Optimise nth_page for contiguous memmap Matthew Wilcox (Oracle)
2021-04-12 6:08 ` Christoph Hellwig
2021-04-09 18:50 ` [PATCH v7 04/28] mm/vmstat: Add functions to account folio statistics Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 05/28] mm/debug: Add VM_BUG_ON_FOLIO and VM_WARN_ON_ONCE_FOLIO Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 07/28] mm: Add put_folio Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 08/28] mm: Add get_folio Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 11/28] mm/filemap: Add folio_index, folio_file_page and folio_contains Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 12/28] mm/filemap: Add folio_next_index Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 15/28] mm: Add folio_mapcount Matthew Wilcox (Oracle)
2021-04-09 18:50 ` [PATCH v7 16/28] mm/memcg: Add folio wrappers for various functions Matthew Wilcox (Oracle)
2021-04-09 18:51 ` [PATCH v7 25/28] mm/writeback: Add wait_for_stable_folio Matthew Wilcox (Oracle)
2021-04-09 18:51 ` [PATCH v7 27/28] mm/filemap: Convert wake_up_page_bit to wake_up_folio_bit Matthew Wilcox (Oracle)
[not found] ` <20210409185105.188284-3-willy@infradead.org>
2021-04-09 22:45 ` [PATCH v7 02/28] mm: Introduce struct folio kernel test robot
2021-04-10 2:43 ` Bogus struct page layout on 32-bit Matthew Wilcox
2021-04-10 6:21 ` Jesper Dangaard Brouer
2021-04-10 8:52 ` Ilias Apalodimas
2021-04-10 14:06 ` Matthew Wilcox
2021-04-10 15:54 ` Russell King - ARM Linux admin
2021-04-16 9:26 ` Grygorii Strashko
2021-04-16 14:10 ` Arnd Bergmann
2021-04-17 13:08 ` David Laight
2021-04-10 14:17 ` David Laight
2021-04-10 19:10 ` Arnd Bergmann [this message]
2021-04-11 22:35 ` Matthew Wilcox
2021-04-10 2:51 ` [PATCH v7 02/28] mm: Introduce struct folio kernel test robot
2021-04-16 15:55 ` Matthew Wilcox
2021-04-19 9:06 ` Kirill A. Shutemov
[not found] ` <20210409185105.188284-10-willy@infradead.org>
2021-04-19 13:25 ` [PATCH v7 09/28] mm: Create FolioFlags Peter Zijlstra
2021-04-19 13:55 ` Matthew Wilcox
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