Re: [PATCH v3 3/3] mm/hmm/test: add self tests for HMM

From: Ralph Campbell <rcampbell@nvidia.com>
To: Jason Gunthorpe <jgg@mellanox.com>
Cc: Jerome Glisse <jglisse@redhat.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Christoph Hellwig <hch@lst.de>,
	"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 3/3] mm/hmm/test: add self tests for HMM
Date: Wed, 30 Oct 2019 17:14:30 -0700	[thread overview]
Message-ID: <f42d06e2-ca08-acdd-948d-2803079a13c2@nvidia.com> (raw)
In-Reply-To: <20191029231255.GX22766@mellanox.com>

On 10/29/19 4:12 PM, Jason Gunthorpe wrote:
> On Tue, Oct 29, 2019 at 02:16:05PM -0700, Ralph Campbell wrote:
> 
>>> Frankly, I'm not super excited about the idea of a 'test driver', it
>>> seems more logical for testing to have some way for a test harness to
>>> call hmm_range_fault() under various conditions and check the results?
>>
>> test_vmalloc.sh at least uses a test module(s).
> 
> Well, that is good, is it also under drivers/char? It kind feels like
> it should not be there...

I think most of the test modules live in lib/ but I wasn't sure that
was the right place for the HMM test driver.
If you think that is better, I can easily move it.

>>> It seems especially over-complicated to use a full page table layout
>>> for this, wouldn't something simple like an xarray be good enough for
>>> test purposes?
>>
>> Possibly. A page table is really just a lookup table from virtual address
>> to pfn/page. Part of the rationale was to mimic what a real device
>> might do.
> 
> Well, but the details of the page table layout don't see really
> important to this testing, IMHO.

One problem with XArray is that on 32-bit machines the value would
need to be u64 to hold a pfn which won't fit in a ULONG_MAX.
I guess we could make the driver 64-bit only.

>>>> +	for (addr = start; addr < end; ) {
>>>> +		long count;
>>>> +
>>>> +		next = min(addr + (ARRAY_SIZE(pfns) << PAGE_SHIFT), end);
>>>> +		range.start = addr;
>>>> +		range.end = next;
>>>> +
>>>> +		down_read(&mm->mmap_sem);
> 
> Also, did we get a mmget() before doing this down_read?
> 
>>>> +
>>>> +		ret = hmm_range_register(&range, &dmirror->mirror);
>>>> +		if (ret) {
>>>> +			up_read(&mm->mmap_sem);
>>>> +			break;
>>>> +		}
>>>> +
>>>> +		if (!hmm_range_wait_until_valid(&range,
>>>> +						DMIRROR_RANGE_FAULT_TIMEOUT)) {
>>>> +			hmm_range_unregister(&range);
>>>> +			up_read(&mm->mmap_sem);
>>>> +			continue;
>>>> +		}
>>>> +
>>>> +		count = hmm_range_fault(&range, 0);
>>>> +		if (count < 0) {
>>>> +			ret = count;
>>>> +			hmm_range_unregister(&range);
>>>> +			up_read(&mm->mmap_sem);
>>>> +			break;
>>>> +		}
>>>> +
>>>> +		if (!hmm_range_valid(&range)) {
>>>
>>> There is no 'driver lock' being held here, how does this work?
>>> Shouldn't it hold dmirror->mutex for this sequence?
>>
>> I have a modified version of this driver that's based on your series
>> removing hmm_mirror_register() which uses a mutex.
>> Otherwise, it looks similar to the changes in nouveau.
> 
> Well, that locking pattern is required even for original hmm calls..

Will be fixed in v4.

> 
>>>> +static int dmirror_read(struct dmirror *dmirror,
>>>> +			struct hmm_dmirror_cmd *cmd)
>>>> +{
>>>
>>> Why not just use pread()/pwrite() for this instead of an ioctl?
>>
>> pread()/pwrite() could certainly be implemented.
>> I think the idea was that the read/write is actually the "device"
>> doing read/write and making that clearly different from a program
>> reading/writing the device. Also, the ioctl() allows information
>> about what faults or events happened during the operation. I only
>> have number of pages and number of page faults returned at the moment,
>> but one of Jerome's version of this driver had other counters being
>> returned.
> 
> Makes sense I guess
> 
>>>> +static struct platform_driver dmirror_device_driver = {
>>>> +	.probe		= dmirror_probe,
>>>> +	.remove		= dmirror_remove,
>>>> +	.driver		= {
>>>> +		.name	= "HMM_DMIRROR",
>>>> +	},
>>>> +};
>>>
>>> This presence of a platform_driver and device is very confusing. I'm
>>> sure Greg KH would object to this as a misuse of platform drivers.
>>>
>>> A platform device isn't needed to create a char dev, so what is this for?
>>
>> The devm_request_free_mem_region() and devm_memremap_pages() calls for
>> creating the ZONE_DEVICE private pages tie into the devm* clean up framework.
>> I thought a platform_driver was the simplest way to also be able to call
>> devm_add_action_or_reset() to clean up on module unload and be compatible
>> with the private page clean up.
> 
> IIRC Christoph recently fixed things so there was a non devm version
> of these functions. Certainly we should not be making fake
> platform_devices just to call devm.
> 
> There is also a struct device inside the cdev, maybe that could be
> arrange to be devm compatible if it was *really* needed.

Will be fixed in v4.

>>>> diff --git a/include/Kbuild b/include/Kbuild
>>>> index ffba79483cc5..6ffb44a45957 100644
>>>> +++ b/include/Kbuild
>>>> @@ -1063,6 +1063,7 @@ header-test-			+= uapi/linux/coda_psdev.h
>>>>    header-test-			+= uapi/linux/errqueue.h
>>>>    header-test-			+= uapi/linux/eventpoll.h
>>>>    header-test-			+= uapi/linux/hdlc/ioctl.h
>>>> +header-test-			+= uapi/linux/hmm_dmirror.h
>>>
>>> Why? This list should only be updated if the header is broken in some
>>> way.
>>
>> Should this be in include/linux/ instead?
>> I wasn't sure where the "right" place was to put the header.
> 
> No, it is right, it just shouldn't be in this makefile.
> 
> Jason

Will be fixed in v4.

Thanks for the review, the code is much simpler now.