Re: [PATCH v2 02/10] block: add helpers for accessing a bio_vec page

In preparation for converting struct bio_vec to carry a __pfn_t instead of struct page. This change is prompted by the desire to add in-kernel DMA support (O_DIRECT, hierarchical storage, RDMA, etc) for persistent memory which lacks struct page coverage. 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. Generated with the following semantic patch: // bv_page.cocci: convert usage of ->bv_page to use set/get helpers // usage: make coccicheck COCCI=bv_page.cocci MODE=patch