On Sat, 10 Apr 2021 21:52:45 +0100
"Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> wrote:
> 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.
>
> We could fix this by telling the compiler to use a smaller alignment
> for the dma_addr, but that seems a little fragile. Instead, move the
> 'flags' into the union. That causes dma_addr to shift into the same
> bits as 'mapping', so it would have to be cleared on free. To avoid
> this, insert three words of padding and use the same bits as ->index
> and ->private, neither of which have to be cleared on free.
>
> Fixes: c25fff7171be ("mm: add dma_addr_t to struct page")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> include/linux/mm_types.h | 38 ++++++++++++++++++++++++++------------
> 1 file changed, 26 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6613b26a8894..45c563e9b50e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -68,16 +68,22 @@ struct mem_cgroup;
> #endif
>
> struct page {
> - unsigned long flags; /* Atomic flags, some possibly
> - * updated asynchronously */
> /*
> - * Five words (20/40 bytes) are available in this union.
> - * WARNING: bit 0 of the first word is used for PageTail(). That
> - * means the other users of this union MUST NOT use the bit to
> + * This union is six words (24 / 48 bytes) in size.
> + * The first word is reserved for atomic flags, often updated
> + * asynchronously. Use the PageFoo() macros to access it. Some
> + * of the flags can be reused for your own purposes, but the
> + * word as a whole often contains other information and overwriting
> + * it will cause functions like page_zone() and page_node() to stop
> + * working correctly.
> + *
> + * Bit 0 of the second word is used for PageTail(). That
> + * means the other users of this union MUST leave the bit zero to
> * avoid collision and false-positive PageTail().
> */
> union {
> struct { /* Page cache and anonymous pages */
> + unsigned long flags;
> /**
> * @lru: Pageout list, eg. active_list protected by
> * lruvec->lru_lock. Sometimes used as a generic list
> @@ -96,13 +102,14 @@ struct page {
> unsigned long private;
> };
> struct { /* page_pool used by netstack */
> - /**
> - * @dma_addr: might require a 64-bit value even on
> - * 32-bit architectures.
> - */
> - dma_addr_t dma_addr;
The original intend of placing member @dma_addr here is that it overlap
with @LRU (type struct list_head) which contains two pointers. Thus, in
case of CONFIG_ARCH_DMA_ADDR_T_64BIT=y on 32-bit architectures it would
use both pointers.
Thinking more about this, this design is flawed as bit 0 of the first
word is used for compound pages (see PageTail and @compound_head), is
reserved. We knew DMA addresses were aligned, thus we though this
satisfied that need. BUT for DMA_ADDR_T_64BIT=y on 32-bit arch the
first word will contain the "upper" part of the DMA addr, which I don't
think gives this guarantee.
I guess, nobody are using this combination?!? I though we added this
to satisfy TI (Texas Instrument) driver cpsw (code in
drivers/net/ethernet/ti/cpsw*). Thus, I assumed it was in use?
> + unsigned long _pp_flags;
> + unsigned long pp_magic;
> + unsigned long xmi;
Matteo notice, I think intent is we can store xdp_mem_info in @xmi.
> + unsigned long _pp_mapping_pad;
> + dma_addr_t dma_addr; /* might be one or two words */
> };
Could you explain your intent here?
I worry about @index.
As I mentioned in other thread[1] netstack use page_is_pfmemalloc()
(code copy-pasted below signature) which imply that the member @index
have to be kept intact. In above, I'm unsure @index is untouched.
[1] https://lore.kernel.org/lkml/20210410082158.79ad09a6@carbon/
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
/*
* Return true only if the page has been allocated with
* ALLOC_NO_WATERMARKS and the low watermark was not
* met implying that the system is under some pressure.
*/
static inline bool page_is_pfmemalloc(const struct page *page)
{
/*
* Page index cannot be this large so this must be
* a pfmemalloc page.
*/
return page->index == -1UL;
}
/*
* Only to be called by the page allocator on a freshly allocated
* page.
*/
static inline void set_page_pfmemalloc(struct page *page)
{
page->index = -1UL;
}
static inline void clear_page_pfmemalloc(struct page *page)
{
page->index = 0;
}
