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

[Luis R. Rodriguez]

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.

>> > > > ioremap() falls back to the cache type of an existing mapping to
>> > > > avoid aliasing.
>> > >
>> > > Does that assume an existing ioremap*() call was used on a bigger
>> > > range?  Do you know if that happens to only be the case for x86 (I'd
>> > > think so) or if its the same for other architectures ?
>> >
>> > In the /dev/mem example, it is remap_pfn_range().  I think other archs
>> > have the same issue, but I do not know if they fall back in case of
>> > overlapping call.
>>
>> What should happen if remap_pfn_range() was used with
>> pgprot_writecombine() and later memremap() is used for instance with a
>> smaller overlappin section, or perhaps bigger?
>
> With the fallback disabled, memremap() should fail in this case.

Excellent.

 Luis
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