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

[Ralph Campbell]

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.