On Mon, 12 Apr 2021 02:15:32 +0100
Matthew Wilcox [off-list ref] wrote:

On Sun, Apr 11, 2021 at 11:33:18AM +0100, Matthew Wilcox wrote:

quoted

Basically, we have three aligned dwords here.  We can either alias with
@flags and the first word of @lru, or the second word of @lru and @mapping,
or @index and @private.  @flags is a non-starter.  If we use @mapping,
then you have to set it to NULL before you free it, and I'm not sure
how easy that will be for you.  If that's trivial, then we could use
the layout:

	unsigned long _pp_flags;
	unsigned long pp_magic;
	union {
		dma_addr_t dma_addr;    /* might be one or two words */
		unsigned long _pp_align[2];
	};
	unsigned long pp_pfmemalloc;
	unsigned long xmi;  

I forgot about the munmap path.  That calls zap_page_range() which calls
set_page_dirty() which calls page_mapping().  If we use page->mapping,
that's going to get interpreted as an address_space pointer.

*sigh*.  Foiled at every turn.

Yes, indeed! - And very frustrating.  It's keeping me up at night.
I'm dreaming about 32 vs 64 bit data structures. My fitbit stats tell
me that I don't sleep well with these kind of dreams ;-)

I'm kind of inclined towards using two (or more) bits for PageSlab as
we discussed here:

https://lore.kernel.org/linux-mm/01000163efe179fe-d6270c58-eaba-482f-a6bd-334667250ef7-000000@email.amazonses.com/ (local)

so we have PageKAlloc that's true for PageSlab, PagePool, PageDMAPool,
PageVMalloc, PageFrag and maybe a few other kernel-internal allocations.

I actually like this idea a lot.  I also think it will solve or remove
Matteo/Ilias'es[2] need to introduce the pp_magic signature.  Ilias do
say[1] that page_pool pages could be used for TCP RX zerocopy, but I
don't think we should "allow" that (meaning page_pool should drop the
DMA-mapping and give up recycling).  I should argue why in that thread.

That said, I think we need to have a quicker fix for the immediate
issue with 64-bit bit dma_addr on 32-bit arch and the misalignment hole
it leaves[3] in struct page.  In[3] you mention ppc32, does it only
happens on certain 32-bit archs?

I'm seriously considering removing page_pool's support for doing/keeping
DMA-mappings on 32-bit arch's.  AFAIK only a single driver use this.

(see also here:)
https://lore.kernel.org/linux-mm/20180518194519.3820-18-willy@infradead.org/ (local)

[1] https://lore.kernel.org/netdev/YHHuE7g73mZNrMV4@enceladus/ (local)
[2] https://patchwork.kernel.org/project/netdevbpf/patch/20210409223801.104657-3-mcroce@linux.microsoft.com/
[3] https://lore.kernel.org/linux-mm/20210410024313.GX2531743@casper.infradead.org/ (local)
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer


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