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

[Jason Gunthorpe]

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...
 
> > 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.

> > > +	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..


> > > +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.

> > > 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