From: Matthew Wilcox <willy@infradead.org>
To: kernel test robot <lkp@intel.com>
Cc: linux-mm@kvack.org, kbuild-all@lists.01.org,
clang-built-linux@googlegroups.com, linux-kernel@vger.kernel.org,
linux-fsdevel@vger.kernel.org,
Michael Ellerman <mpe@ellerman.id.au>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org,
Jesper Dangaard Brouer <brouer@redhat.com>,
"David S. Miller" <davem@davemloft.net>
Subject: Bogus struct page layout on 32-bit
Date: Sat, 10 Apr 2021 03:43:13 +0100 [thread overview]
Message-ID: <20210410024313.GX2531743@casper.infradead.org> (raw)
In-Reply-To: <202104100656.N7EVvkNZ-lkp@intel.com>
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.
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;
/**
* @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?
next prev parent reply other threads:[~2021-04-10 2:44 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 ` Matthew Wilcox [this message]
2021-04-10 6:21 ` Bogus struct page layout on 32-bit 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
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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