linux-kernel - Re: [PATCH v2 08/25] arch: introduce memremap()

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Message-ID: <CAPcyv4jN6jYzw11qOOR-xd_w6s5dguQSoHyvmPFSWtL9S+tz3w@mail.gmail.com>
Date:	Mon, 27 Jul 2015 16:26:03 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	"Kani, Toshimitsu" <toshi.kani@...com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 08/25] arch: introduce memremap()

On Sun, Jul 26, 2015 at 10:12 PM, Christoph Hellwig <hch@....de> wrote:
> On Sun, Jul 26, 2015 at 10:49:39AM -0700, Dan Williams wrote:
>> On Sun, Jul 26, 2015 at 10:25 AM, Christoph Hellwig <hch@....de> wrote:
>> > On Fri, Jul 24, 2015 at 10:38:42PM -0400, Dan Williams wrote:
>> >> The behavior change to return NULL on an unsupported request is reserved
>> >> for a later patch.
>> >
>> > Why?
>>
>> This is for drivers like pmem that care about the mapping type.  For
>> example, if pmem can't get a cache-enabled mapping it is potentially
>> putting the write durability of the persistent media at risk.
>
> I understand that part, but the question is why the old behavior
> is retained for now and only changed later.

Oh, because all we have at this point is ioremap_cache() which
silently falls back.  It's not until the introduction of
arch_memremp() where we update the arch code to break that behavior.

That said, I think it may be beneficial to allow a fallback if the
user cares.  So maybe memremap() can call plain ioremap() if
MEMREMAP_STRICT is not set and none of the other mapping types are
satisfied.
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