On Wed, Feb 10, 2021 at 02:40:10PM +1100, Alistair Popple wrote:
On Wednesday, 10 February 2021 12:39:32 AM AEDT Jason Gunthorpe wrote:
On Tue, Feb 09, 2021 at 12:07:14PM +1100, Alistair Popple wrote:
Device private pages are used to represent device memory that is not directly accessible from the CPU. Extra references to a device private page are only used to ensure the struct page itself remains valid whilst waiting for migration entries. Therefore extra references should not prevent device private page migration as this can lead to failures to migrate pages back to the CPU which are fatal to the user process.
This should identify the extra references in expected_count, just disabling this protection seems unsafe, ZONE_DEVICE is not so special that the refcount means nothing
This is similar to what migarte_vma_check_page() does now. The issue is that a migration wait takes a reference on the device private page so you can end up with one thread stuck waiting for migration whilst the other can't migrate due to the extra refcount.
Given device private pages can't undergo GUP and that it's not possible to differentiate the migration wait refcount from any other refcount we assume any possible extra reference must be from migration wait.
GUP is not the only thing that elevates the refcount, I think this is an unsafe assumption
Why is migration holding an extra refcount anyhow?
Jason