On 8/16/19 10:28 AM, Jason Gunthorpe wrote:
On Fri, Aug 16, 2019 at 10:21:41AM -0700, Dan Williams wrote:
We can do a get_dev_pagemap inside the page_walk and touch the pgmap,
or we can do the 'device mutex && retry' pattern and touch the pgmap
in the driver, under that lock.
However in all cases the current get_dev_pagemap()'s in the page walk
are not necessary, and we can delete them.
Yes, as long as 'struct page' instances resulting from that lookup are
not passed outside of that lock.
Indeed.
Also, I was reflecting over lunch that the hmm_range_fault should only
return DEVICE_PRIVATE pages for the caller's device (see other thread
with HCH), and in this case, the caller should also be responsible to
ensure that the driver is not calling hmm_range_fault at the same time
it is deleting it's own DEVICE_PRIVATE mapping - ie by fencing its
page fault handler.
Yes, that would make it a one step process to access another
device's migrated memory pages.
Right now, it has to be a two step process where the caller calls
hmm_range_fault, check the struct page to see if it is device
private and not owned, then call hmm_range_fault again with
range->pfns[i] |= range->flags[HMM_PFN_DEVICE_PRIVATE] to cause
the other device to migrate the page back to system memory.
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