Re: [PATCH -v2] Resource: fix region_intersects() for CXL memory

["Huang, Ying" <ying.huang@intel.com>]

Hi, Andy,

Thanks for your comments!

Andy Shevchenko <andriy.shevchenko@linux.intel.com> writes:

> 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)
>> related to CXL memory looks like something as follows.
>> 
>> 490000000-50fffffff : CXL Window 0
>>   490000000-50fffffff : region0
>>     490000000-50fffffff : dax0.0
>>       490000000-50fffffff : System RAM (kmem)
>> 
>> When the following command line is run to try writing some memory in
>> CXL memory range,
>> 
>>  $ dd if=data of=/dev/mem bs=1k seek=19136512 count=1
>>  dd: error writing '/dev/mem': Bad address
>>  1+0 records in
>>  0+0 records out
>>  0 bytes copied, 0.0283507 s, 0.0 kB/s
>> 
>> the command fails as expected.  However, the error code is wrong.  It
>> should be "Operation not permitted" instead of "Bad address".  And,
>> the following warning is reported in kernel log.
>
>>  ioremap on RAM at 0x0000000490000000 - 0x0000000490000fff
>>  WARNING: CPU: 2 PID: 416 at arch/x86/mm/ioremap.c:216 __ioremap_caller.constprop.0+0x131/0x35d
>>  Modules linked in: cxl_pmem libnvdimm cbc encrypted_keys cxl_pmu
>>  CPU: 2 UID: 0 PID: 416 Comm: dd Not tainted 6.11.0-rc3-kvm #40
>>  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
>>  RIP: 0010:__ioremap_caller.constprop.0+0x131/0x35d
>>  Code: 2d 80 3d 24 6a a1 02 00 75 c1 48 8d 54 24 70 48 8d b4 24 90 00 00 00 48 c7 c7 40 3a 05 8a c6 05 07 6a a1 02 01 e8 a3 a0 01 00 <0f> 0b eb 9d 48 8b 84 24 90 00 00 00 48 8d 4c 24 60 89 ea 48 bf 00
>>  RSP: 0018:ffff888105387bf0 EFLAGS: 00010282
>>  RAX: 0000000000000000 RBX: 0000000490000fff RCX: 0000000000000000
>>  RDX: 0000000000000001 RSI: 0000000000000003 RDI: ffffed1020a70f73
>>  RBP: 0000000000000000 R08: ffffed100d9fce92 R09: 0000000000000001
>>  R10: ffffffff892348e7 R11: ffffed100d9fce91 R12: 0000000490000000
>>  R13: 0000000000000001 R14: 0000000000000001 R15: ffff888105387ca0
>>  FS:  00007f86c438c740(0000) GS:ffff88806ce00000(0000) knlGS:0000000000000000
>>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>  CR2: 000055ba75b1b818 CR3: 0000000005231000 CR4: 0000000000350eb0
>>  Call Trace:
>>   <TASK>
>>   ? __warn+0xd7/0x1b8
>>   ? __ioremap_caller.constprop.0+0x131/0x35d
>>   ? report_bug+0x136/0x19e
>>   ? __ioremap_caller.constprop.0+0x131/0x35d
>>   ? handle_bug+0x3c/0x64
>>   ? exc_invalid_op+0x13/0x38
>>   ? asm_exc_invalid_op+0x16/0x20
>>   ? irq_work_claim+0x16/0x38
>>   ? __ioremap_caller.constprop.0+0x131/0x35d
>>   ? tracer_hardirqs_off+0x18/0x16d
>>   ? kmem_cache_debug_flags+0x16/0x23
>>   ? memremap+0xcb/0x184
>>   ? iounmap+0xfe/0xfe
>>   ? lock_sync+0xc7/0xc7
>>   ? lock_sync+0xc7/0xc7
>>   ? rcu_is_watching+0x1c/0x38
>>   ? do_raw_read_unlock+0x37/0x42
>>   ? _raw_read_unlock+0x1f/0x2f
>>   memremap+0xcb/0x184
>>   ? pfn_valid+0x159/0x159
>>   ? resource_is_exclusive+0xba/0xc5
>>   xlate_dev_mem_ptr+0x25/0x2f
>>   write_mem+0x94/0xfb
>>   vfs_write+0x128/0x26d
>>   ? kernel_write+0x89/0x89
>>   ? rcu_is_watching+0x1c/0x38
>>   ? __might_fault+0x72/0xba
>>   ? __might_fault+0x72/0xba
>>   ? rcu_is_watching+0x1c/0x38
>>   ? lock_release+0xba/0x13e
>>   ? files_lookup_fd_raw+0x40/0x4b
>>   ? __fget_light+0x64/0x89
>>   ksys_write+0xac/0xfe
>>   ? __ia32_sys_read+0x40/0x40
>>   ? tracer_hardirqs_off+0x18/0x16d
>>   ? tracer_hardirqs_on+0x11/0x146
>>   do_syscall_64+0x9a/0xfd
>>   entry_SYSCALL_64_after_hwframe+0x4b/0x53
>>  RIP: 0033:0x7f86c4487140
>>  Code: 40 00 48 8b 15 c1 9c 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 80 3d a1 24 0e 00 00 74 17 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 48 83 ec 28 48 89
>>  RSP: 002b:00007ffca9f62af8 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
>>  RAX: ffffffffffffffda RBX: 0000000000000400 RCX: 00007f86c4487140
>>  RDX: 0000000000000400 RSI: 000055ba75b1a000 RDI: 0000000000000001
>>  RBP: 000055ba75b1a000 R08: 0000000000000000 R09: 00007f86c457c080
>>  R10: 00007f86c43a84d0 R11: 0000000000000202 R12: 0000000000000000
>>  R13: 0000000000000000 R14: 000055ba75b1a000 R15: 0000000000000400
>>   </TASK>
>>  irq event stamp: 0
>>  hardirqs last  enabled at (0): [<0000000000000000>] 0x0
>>  hardirqs last disabled at (0): [<ffffffff89091e85>] copy_process+0xb60/0x255f
>>  softirqs last  enabled at (0): [<ffffffff89091e85>] copy_process+0xb60/0x255f
>>  softirqs last disabled at (0): [<0000000000000000>] 0x0
>
> Submitting Patches documentation suggests how to shrink the above to make it
> somewhat readable and important.

Thanks for the pointer.  Will do that in the next version.

>> After investigation, we found the following bug.
>> 
>> 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" --|
>> 
>> in effect.
>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Davidlohr Bueso <dave@stgolabs.net>
>> Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
>> Cc: Dave Jiang <dave.jiang@intel.com>
>> Cc: Alison Schofield <alison.schofield@intel.com>
>> Cc: Vishal Verma <vishal.l.verma@intel.com>
>> Cc: Ira Weiny <ira.weiny@intel.com>
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
>> Cc: Bjorn Helgaas <bhelgaas@google.com>
>> Cc: Baoquan He <bhe@redhat.com>
>
> You may move Cc list after '---', so it won't unnecessarily pollute the commit
> message.

Emm... It appears that it's a common practice to include "Cc" in the
commit log.

>> ---
>> Changelogs:
>> 
>> v2:
>> 
>> - Fix a bug which occurs when descendant of a matched resource matches.
>> 
>> - Revise the patch description and comments to make the algorithm clearer.
>>   Thanks Dan and David's comments!
>> 
>> - Link to v1: https://lore.kernel.org/linux-mm/20240816020723.771196-1-ying.huang@intel.com/
>
> ...
>
>>  {
>>  	struct resource res;
>>  	int type = 0; int other = 0;
>> -	struct resource *p;
>> +	struct resource *p, *dp;
>> +	resource_size_t ostart, oend;
>> +	bool is_type, covered;
>
> Maybe keep more reversed xmas tree ordering?

OK.

>>  	res.start = start;
>>  	res.end = start + size - 1;
>>  
>>  	for (p = parent->child; p ; p = p->sibling) {
>> -		bool is_type = (((p->flags & flags) == flags) &&
>> -				((desc == IORES_DESC_NONE) ||
>> -				 (desc == p->desc)));
>> -
>> -		if (resource_overlaps(p, &res))
>> -			is_type ? type++ : other++;
>> +		if (!resource_overlaps(p, &res))
>> +			continue;
>> +		is_type = (((p->flags & flags) == flags) &&
>> +			   ((desc == IORES_DESC_NONE) || (desc == p->desc)));
>
> In the original code and here the most outer parentheses are redundant.

OK.  Will remove them.

>> +		if (is_type) {
>> +			type++;
>> +			continue;
>> +		}
>> +		/*
>> +		 * Continue to search in descendant resources as if the
>> +		 * matched descendant resources cover some ranges of 'p'.
>> +		 *
>> +		 * |------------- "CXL Window 0" ------------|
>> +		 * |-- "System RAM" --|
>> +		 *
>> +		 * looks as if
>> +		 *
>> +		 * |-- "System RAM" --||-- "CXL Window 0a" --|
>> +		 *
>> +		 * in effect.
>> +		 */
>> +		covered = false;
>
>> +		ostart = max(res.start, p->start);
>> +		oend = min(res.end, p->end);
>
> Isn't a reinvention of resource_intersection()? With that in place you may also
> drop the above resource_overlaps().

sizeof(struct resource) == 8 * sizeof(unsigned long)

Just want to avoid to define another struct resource on stack.

>> +		for_each_resource(p, dp, false) {
>> +			if (!resource_overlaps(dp, &res))
>> +				continue;
>> +			is_type = (((dp->flags & flags) == flags) &&
>> +				   ((desc == IORES_DESC_NONE) ||
>> +				    (desc == dp->desc)));
>
> Most outer parentheses are redundant. With them being dropped you also may
> unite the last two lines into a single one.

Sure.  Will do that.

>> +			if (is_type) {
>> +				type++;
>> +				if (dp->start > ostart)
>> +					break;
>> +				if (dp->end >= oend) {
>> +					covered = true;
>> +					break;
>> +				}
>> +				ostart = max(ostart, dp->end + 1);
>> +			}
>> +		}
>> +		if (!covered)
>> +			other++;
>>  	}

--
Best Regards,
Huang, Ying