Re: [PATCH 2/3] mm: drop VMA lock before waiting for migration

[Matthew Wilcox <willy@infradead.org>]

On Mon, May 01, 2023 at 10:50:24AM -0700, Suren Baghdasaryan wrote:
> migration_entry_wait does not need VMA lock, therefore it can be dropped
> before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA
> lock was dropped while in handle_mm_fault().
> Note that once VMA lock is dropped, the VMA reference can't be used as
> there are no guarantees it was not freed.

How about we introduce:

void vmf_end_read(struct vm_fault *vmf)
{
	if (!vmf->vma)
		return;
	vma_end_read(vmf->vma);
	vmf->vma = NULL;
}

Now we don't need a new flag, and calling vmf_end_read() is idempotent.

Oh, argh, we create the vmf too late.  We really need to hoist the
creation of vm_fault to the callers of handle_mm_fault().