Re: [PATCH] KVM: use set_page_dirty rather than SetPageDirty

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 26, 2022, Boris Burkov wrote:
> I tested this fix on the workload and it did prevent the hangs. However,
> I am unsure if the fix is appropriate from a locking perspective, so I
> hope to draw some extra attention to that aspect. set_page_dirty_lock in
> mm/page-writeback.c has a comment about locking that says set_page_dirty
> should be called with the page locked or while definitely holding a
> reference to the mapping's host inode. I believe that the mmap should
> have that reference, so for fear of hurting KVM performance or
> introducing a deadlock, I opted for the unlocked variant.

KVM doesn't hold a reference per se, but it does subscribe to mmu_notifier events
and will not mark the page dirty after KVM has been instructed to unmap the page
(barring bugs, which we've had a slew of).  So yeah, the unlocked variant should
be safe.

Is it feasible to trigger this behavior in a selftest?  KVM has had, and probably
still has, many bugs that all boil down to KVM assuming guest memory is backed by
either anonymous memory or something like shmem/HugeTLBFS/memfd that isn't typically
truncated by the host.