linux-kernel - Re: [PATCH v2] x86, uaccess: introduce copy_from_iter

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Date:   Fri, 5 May 2017 08:54:04 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     viro@...iv.linux.org.uk, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, hch@....de,
        linux-block@...r.kernel.org, linux-nvdimm@...ts.01.org,
        jmoyer@...hat.com, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-fsdevel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        ross.zwisler@...ux.intel.com
Subject: Re: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem /
 writethrough operations


* Dan Williams <dan.j.williams@...el.com> wrote:

> The pmem driver has a need to transfer data with a persistent memory
> destination and be able to rely on the fact that the destination writes
> are not cached. It is sufficient for the writes to be flushed to a
> cpu-store-buffer (non-temporal / "movnt" in x86 terms), as we expect
> userspace to call fsync() to ensure data-writes have reached a
> power-fail-safe zone in the platform. The fsync() triggers a REQ_FUA or
> REQ_FLUSH to the pmem driver which will turn around and fence previous
> writes with an "sfence".
> 
> Implement a __copy_from_user_inatomic_wt, memcpy_page_wt, and memcpy_wt,
> that guarantee that the destination buffer is not dirty in the cpu cache
> on completion. The new copy_from_iter_wt and sub-routines will be used
> to replace the "pmem api" (include/linux/pmem.h +
> arch/x86/include/asm/pmem.h). The availability of copy_from_iter_wt()
> and memcpy_wt() are gated by the CONFIG_ARCH_HAS_UACCESS_WT config
> symbol, and fallback to copy_from_iter_nocache() and plain memcpy()
> otherwise.
> 
> This is meant to satisfy the concern from Linus that if a driver wants
> to do something beyond the normal nocache semantics it should be
> something private to that driver [1], and Al's concern that anything
> uaccess related belongs with the rest of the uaccess code [2].
> 
> [1]: https://lists.01.org/pipermail/linux-nvdimm/2017-January/008364.html
> [2]: https://lists.01.org/pipermail/linux-nvdimm/2017-April/009942.html
> 
> Cc: <x86@...nel.org>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Jeff Moyer <jmoyer@...hat.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Matthew Wilcox <mawilcox@...rosoft.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
> Changes since the initial RFC:
> * s/writethru/wt/ since we already have ioremap_wt(), set_memory_wt(),
>   etc. (Ingo)

Looks good to me. I suspect you'd like to carry this in the nvdimm tree?

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo