Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, May 06, 2015 at 04:04:53PM -0400, Dan Williams wrote:
> Changes since v1 [1]:
> 
> 1/ added include/asm-generic/pfn.h for the __pfn_t definition and helpers.
> 
> 2/ added kmap_atomic_pfn_t()
> 
> 3/ rebased on v4.1-rc2
> 
> [1]: http://marc.info/?l=linux-kernel&m=142653770511970&w=2
> 
> ---
> 
> A lead in note, this looks scarier than it is.  Most of the code thrash
> is automated via Coccinelle.  Also the subtle differences behind an
> 'unsigned long pfn' and a '__pfn_t' are mitigated by type-safety and a
> Kconfig option (default disabled CONFIG_PMEM_IO) that globally controls
> whether a pfn and a __pfn_t are equivalent.
> 
> The motivation for this change is persistent memory and the desire to
> use it not only via the pmem driver, but also as a memory target for I/O
> (DAX, O_DIRECT, DMA, RDMA, etc) in other parts of the kernel.  Aside
> from the pmem driver and DAX, persistent memory is not able to be used
> in these I/O scenarios due to the lack of a backing struct page, i.e.
> persistent memory is not part of the memmap.  This patchset takes the
> position that the solution is to teach I/O paths that want to operate on
> persistent memory to do so by referencing a __pfn_t.  The alternatives
> are discussed in the changelog for "[PATCH v2 01/10] arch: introduce
> __pfn_t for persistent memory i/o", copied here:
> 
>     Alternatives:
>     
>     1/ Provide struct page coverage for persistent memory in
>        DRAM.  The expectation is that persistent memory capacities make
>        this untenable in the long term.
>     
>     2/ Provide struct page coverage for persistent memory with
>        persistent memory.  While persistent memory may have near DRAM
>        performance characteristics it may not have the same
>        write-endurance of DRAM.  Given the update frequency of struct
>        page objects it may not be suitable for persistent memory.
>     
>     3/ Dynamically allocate struct page.  This appears to be on
>        the order of the complexity of converting code paths to use
>        __pfn_t references instead of struct page, and the amount of
>        setup required to establish a valid struct page reference is
>        mostly wasted when the only usage in the block stack is to
>        perform a page_to_pfn() conversion for dma-mapping.  Instances
>        of kmap() / kmap_atomic() usage appear to be the only occasions
>        in the block stack where struct page is non-trivially used.  A
>        new kmap_atomic_pfn_t() is proposed to handle those cases.

*grumble*

What are you going to do with things like iov_iter_get_pages()?  Long-term,
that is, after you go for "this pfn has no struct page for it"...