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Message-ID: <20150506205033.GA889@ZenIV.linux.org.uk>
Date: Wed, 6 May 2015 21:50:33 +0100
From: Al Viro <viro@...IV.linux.org.uk>
To: Dan Williams <dan.j.williams@...el.com>
Cc: linux-kernel@...r.kernel.org, Boaz Harrosh <boaz@...xistor.com>,
Jan Kara <jack@...e.cz>, Mike Snitzer <snitzer@...hat.com>,
Neil Brown <neilb@...e.de>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Chris Mason <clm@...com>, Paul Mackerras <paulus@...ba.org>,
"H. Peter Anvin" <hpa@...or.com>, hch@....de,
Alasdair Kergon <agk@...hat.com>, linux-nvdimm@...1.01.org,
mingo@...nel.org, mgorman@...e.de,
Matthew Wilcox <willy@...ux.intel.com>,
Ross Zwisler <ross.zwisler@...ux.intel.com>, riel@...hat.com,
Martin Schwidefsky <schwidefsky@...ibm.com>, axboe@...nel.dk,
Theodore Ts'o <tytso@....edu>,
"Martin K. Petersen" <martin.petersen@...cle.com>,
Julia Lawall <Julia.Lawall@...6.fr>, Tejun Heo <tj@...nel.org>,
linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer,
introduce __pfn_t
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"...
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