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

[Dan Williams]

On Tue, Aug 11, 2015 at 3:52 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> On Tue, Aug 11, 2015 at 3:40 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
>> On Tue, 2015-08-11 at 23:30 +0200, Luis R. Rodriguez wrote:
>>> On Wed, Jul 29, 2015 at 06:00:04PM -0600, Toshi Kani wrote:
>>> > On Wed, 2015-07-29 at 23:43 +0200, Luis R. Rodriguez wrote:
>>> > > On Wed, Jul 29, 2015 at 03:00:38PM -0600, Toshi Kani wrote:
>>> > > > On Wed, 2015-07-29 at 11:33 -0700, Dan Williams wrote:
>>   :
>>> > > That would depend on the purpose of the region and the driver
>>> > > developer should in theory know best. One issue with this of course is
>>> > > that, as we've discovered, the semantics of on the ioremap*() variant
>>> > > front regarding cache types is not clearly well defined, or at least
>>> > > it may be only well defined implicitly on x86 only, so the driver
>>> > > developer can only *hope* for the best across architectures. Our
>>> > > ambiguity on our semantics on ioremap*() variants therefore means
>>> > > driver developers can resonably be puzzled by what fallbacks to use.
>>> > > That also means architectures maintainers should not whip driver
>>> > > developers for any misuse. Such considerations are why although we're
>>> > > now revisiting semantics for ioremap*() variants I was in hopes we
>>> > > could be at least somewhat pedantic about memremap() semantics.
>>> >
>>> > I agree.  However, there are a few exceptions like /dev/mem, which can
>>> > map a target range without knowledge.
>>>
>>> Still, the expectation to require support for overlapping ioremap() calls
>>> would seem to be more of an exception than the norm, so I'd argue that
>>> requiring callers who know they do need overlapping support to be explicit
>>> about it may help us simplify expecations on semantics in other areas of
>>> the kernel.
>>
>> Again, I agree.  I am simply saying that the fallback in an overlapping case
>> may need to remain supported for such exceptional cases, possibly with a
>> separate interface.
>
> Great.
>
>>> > > For instance since memremap() only has 2 types right now can a
>>> > > respective fallback be documented as an alternative to help with this
>>> > > ? Or can we not generalize this ?  One for MEMREMAP_WB and one for
>>> > > MEMREMAP_WT ?
>>> >
>>> > Yes, if a target range can be only mapped by memremap().  However, there
>>> > is no restriction that a range can be mapped with a single interface
>>> > alone.  For example, a range can be mapped with remap_pfn_range() to
>>> > user space with any cache type.  So, in theory, memremap() can overlap
>>> > with any other types.
>>>
>>> Shouldn't that be an issue or area of concern ? It seems the flakiness on
>>> ioremap() and its wide array flexibility would spill over the any
>>> semantics which folks would be trying to set out with memremap(). That
>>> should not stop the evolution of memremap() though, just pointing out that
>>> perhaps we should be a bit more restrictive over how things can criss
>>> -cross and who areas can do it.
>>
>> reserve_pfn_range() allows the caller to specify if the fallback needs to be
>> enabled or disabled with 'strict_prot'.  track_pfn_remap() called from
>> remap_pfn_range() enables it, and track_pfn_copy() disables it.  I think we
>> can do similar for the memremap and ioremap families as well.  The fallback
>> could be set disabled in the regular interfaces, and enabled in some
>> interface as necessary.  This also allows gradual transition, ex. disable in
>> memremap while ioremap remains enabled for now.
>
> Sounds sexy to me.

Cool, sounds like something we can tackle in 4.4 along with the
ioremap_cache removal cleanups.
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