On 3/24/21 2:48 PM, Jason Gunthorpe wrote:
On Wed, Mar 24, 2021 at 02:35:38PM +0100, Thomas Hellström (Intel) wrote:
In an ideal world the creation/destruction of page table levels would
by dynamic at this point, like THP.
Hmm, but I'm not sure what problem we're trying to solve by changing the
interface in this way?
We are trying to make a sensible driver API to deal with huge pages.
Currently if the core vm requests a huge pud, we give it one, and if we
can't or don't want to (because of dirty-tracking, for example, which is
always done on 4K page-level) we just return VM_FAULT_FALLBACK, and the
fault is retried at a lower level.
Well, my thought would be to move the pte related stuff into
vmf_insert_range instead of recursing back via VM_FAULT_FALLBACK.
I don't know if the locking works out, but it feels cleaner that the
driver tells the vmf how big a page it can stuff in, not the vm
telling the driver to stuff in a certain size page which it might not
want to do.
Some devices want to work on a in-between page size like 64k so they
can't form 2M pages but they can stuff 64k of 4K pages in a batch on
every fault.
Hmm, yes, but we would in that case be limited anyway to insert ranges
smaller than and equal to the fault size to avoid extensive and possibly
unnecessary checks for contigous memory. And then if we can't support
the full fault size, we'd need to either presume a size and alignment of
the next level or search for contigous memory in both directions around
the fault address, perhaps unnecessarily as well. I do think the current
interface works ok, as we're just acting on what the core vm tells us to do.
/Thomas
That idea doesn't fit naturally if the VM is driving the size.
Jason
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