From: Geert Uytterhoeven

Sent: 20 April 2021 08:40

Hi Willy,

On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox [off-list ref] wrote:

quoted

Replacement patch to fix compiler warning.

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

--- 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.

You could use [ARRAY_SIZE()-1] instead of [1].
Or, since IIRC it is the last member of that specific struct, define as:
		unsigned long dma_addr[];

...

quoted

-       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;

Or:
	ret |= page->dma_addr[1] + 0ull << 32;

Which relies in integer promotion rather than a cast.

	David

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