linux-kernel - Re: [PATCH v5 6/6] arch, x86: pmem api for ensuring durability of persistent memory updates

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Date:	Mon, 22 Jun 2015 10:51:57 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Juergen Gross <jgross@...e.com>, X86 ML <x86@...nel.org>,
	"Kani, Toshimitsu" <toshi.kani@...com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Luis Rodriguez <mcgrof@...e.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefan Bader <stefan.bader@...onical.com>,
	Andy Lutomirski <luto@...capital.net>, linux-mm@...ck.org,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	mpe@...erman.id.au, Tejun Heo <tj@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Richard Weinberger <richard@....at>
Subject: Re: [PATCH v5 6/6] arch, x86: pmem api for ensuring durability of
 persistent memory updates

On Mon, Jun 22, 2015 at 9:17 AM, Christoph Hellwig <hch@....de> wrote:
>> +#ifdef ARCH_HAS_NOCACHE_UACCESS
>
> Seems like this is always define for x86 anyway?
>
>> +/**
>> + * arch_memcpy_to_pmem - copy data to persistent memory
>> + * @dst: destination buffer for the copy
>> + * @src: source buffer for the copy
>> + * @n: length of the copy in bytes
>> + *
>> + * Copy data to persistent memory media via non-temporal stores so that
>> + * a subsequent arch_wmb_pmem() can flush cpu and memory controller
>> + * write buffers to guarantee durability.
>> + */
> static inline void arch_memcpy_to_pmem(void __pmem *dst, const void *src, size_t n)
>
> Too long line.  Also why not simply arch_copy_{from,to}_pmem?

I'm following the precedence set by memcpy_{from,to}_io().

>> +static inline void __pmem *arch_memremap_pmem(resource_size_t offset,
>> +     unsigned long size)
>> +{
>> +     return (void __force __pmem *) ioremap_cache(offset, size);
>> +}
>
> Now with my ioremap_flags proposal we'd just add an IOREMAP_PMEM
> flag, which architectures could implement (usually as no-op), and move
> the cast into memremap_pmem.

*nod*

>
>> + * These defaults seek to offer decent performance and minimize the
>> + * window between i/o completion and writes being durable on media.
>> + * However, it is undefined / architecture specific whether
>> + * default_memremap_pmem + default_memcpy_to_pmem is sufficient for
>> + * making data durable relative to i/o completion.
>> + */
>> +static void default_memcpy_to_pmem(void __pmem *dst, const void *src, size_t size)
>> +{
>> +     memcpy((void __force *) dst, src, size);
>> +}
>
> This should really be in asm-generic (or at least your linux/pmem.h for now).

ok.

>> +static void __pmem *default_memremap_pmem(resource_size_t offset, unsigned long size)
>> +{
>> +     return (void __pmem *)memremap_wt(offset, size);
>> +}
>
> And this as well, unless we can get rid of it entirely with ioremap_flags().

I'll move it for now.  ioremap_flags() requires more care than can be
given in the open merge window as far as I can see.

>>       if (rw == READ) {
>> -             memcpy(mem + off, pmem->virt_addr + pmem_off, len);
>> +             memcpy_from_pmem(mem + off, pmem_addr, len);
>>               flush_dcache_page(page);
>>       } else {
>>               flush_dcache_page(page);
>> -             memcpy(pmem->virt_addr + pmem_off, mem + off, len);
>> +             if (arch_has_pmem_api())
>> +                     memcpy_to_pmem(pmem_addr, mem + off, len);
>> +             else
>> +                     default_memcpy_to_pmem(pmem_addr, mem + off, len);
>
> So memcpy_from_pmem hides the different but memcpy_to_pmem doesn't?
> That seems pretty awkward.  Please move the check into the helper.

ok

>> +     if (rw && arch_has_pmem_api())
>> +             wmb_pmem();
>
> And here again make sure wmb_pmem is always available and a no-op if
> not supported.

ok.

>
>> +     if (arch_has_pmem_api())
>> +             pmem->virt_addr = memremap_pmem(pmem->phys_addr, pmem->size);
>> +     else
>> +             pmem->virt_addr = default_memremap_pmem(pmem->phys_addr,
>> +                             pmem->size);
>
> All of this should be hidden in memremap_pmem.

done.
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