linux-kernel - Re: [PATCH -v2] Resource: fix region

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Message-ID: <87ikvih5nv.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Fri, 30 Aug 2024 14:43:32 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Bjorn Helgaas <helgaas@...nel.org>,  Andrew Morton
 <akpm@...ux-foundation.org>,  linux-mm@...ck.org,
  linux-kernel@...r.kernel.org,  linux-cxl@...r.kernel.org,  David
 Hildenbrand <david@...hat.com>,  Davidlohr Bueso <dave@...olabs.net>,
  Jonathan Cameron <jonathan.cameron@...wei.com>,  Dave Jiang
 <dave.jiang@...el.com>,  Alison Schofield <alison.schofield@...el.com>,
  Vishal Verma <vishal.l.verma@...el.com>,  Ira Weiny
 <ira.weiny@...el.com>,  Alistair Popple <apopple@...dia.com>,  Andy
 Shevchenko <andriy.shevchenko@...ux.intel.com>,  Bjorn Helgaas
 <bhelgaas@...gle.com>,  Baoquan He <bhe@...hat.com>
Subject: Re: [PATCH -v2] Resource: fix region_intersects() for CXL memory

Hi, Dan,

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

> Hi Bjorn,
>
> Ying is out for the next week or so, so I will address your comments and
> resubmit as I think this is a potentially urgent fix.

Just come back.  Thank you very much for your kind help!

> Bjorn Helgaas wrote:
>> On Mon, Aug 19, 2024 at 10:34:13AM +0800, Huang Ying wrote:
>> > On a system with CXL memory installed, the resource tree (/proc/iomem)

[snip]

>> > In the above resource tree, "System RAM" is a descendant of "CXL
>> > Window 0" instead of a top level resource.  So, region_intersects()
>> > will report no System RAM resources in the CXL memory region
>> > incorrectly, because it only checks the top level resources.
>> > Consequently, devmem_is_allowed() will return 1 (allow access via
>> > /dev/mem) for CXL memory region incorrectly.  Fortunately, ioremap()
>> > doesn't allow to map System RAM and reject the access.
>> > 
>> > However, region_intersects() needs to be fixed to work correctly with
>> > the resources tree with CXL Window as above.  To fix it, if we found a
>> > unmatched resource in the top level, we will continue to search
>> > matched resources in its descendant resources.  So, we will not miss
>> > any matched resources in resource tree anymore.  In the new
>> > implementation,
>> > 
>> > |------------- "CXL Window 0" ------------|
>> > |-- "System RAM" --|
>> > 
>> > will look as if
>> > 
>> > |-- "System RAM" --||-- "CXL Window 0a" --|
>> 
>> Where did "0a" come from?  The /proc/iomem above mentioned
>> "CXL Window 0"; is the "a" spurious?  Same question applies to the
>> code comment below.
>
> Not sure where that came from, will clean up and provide a test that can
> upstreammed as well.

Sorry for confusing!  

>> > |-- "System RAM" --||-- "CXL Window 0a" --|

This isn't the real resource tree.  The real resource tree is still

>> > |------------- "CXL Window 0" ------------|
>> > |-- "System RAM" --|

While,

>> > |-- "System RAM" --||-- "CXL Window 0a" --|

is just used to demonstrate the effect of the algorithm.  That is, a
part of "CXL Window 0" is covered (or masked) by "System RAM" now.  The
remaining effective part of "CXL Window 0" is smaller, I used "CXL
Window 0a" to designate that it comes from "CXL Window 0", but it's not
exactly "CXL Window 0".

--
Best Regards,
Huang, Ying