Re: [PATCH v3 7/8] mm: drop VMA lock before waiting for migration

[Peter Xu <peterx@redhat.com>]

On Mon, Jun 26, 2023 at 09:23:20PM -0700, Suren Baghdasaryan wrote:
> migration_entry_wait does not need VMA lock, therefore it can be
> dropped before waiting.

Hmm, I'm not sure..

Note that we're still dereferencing *vmf->pmd when waiting, while *pmd is
on the page table and IIUC only be guaranteed if the vma is still there.
If without both mmap / vma lock I don't see what makes sure the pgtable is
always there.  E.g. IIUC a race can happen where unmap() runs right after
vma_end_read() below but before pmdp_get_lockless() (inside
migration_entry_wait()), then pmdp_get_lockless() can read some random
things if the pgtable is freed.

> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  mm/memory.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 5caaa4c66ea2..bdf46fdc58d6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3715,8 +3715,18 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	entry = pte_to_swp_entry(vmf->orig_pte);
>  	if (unlikely(non_swap_entry(entry))) {
>  		if (is_migration_entry(entry)) {
> -			migration_entry_wait(vma->vm_mm, vmf->pmd,
> -					     vmf->address);
> +			/* Save mm in case VMA lock is dropped */
> +			struct mm_struct *mm = vma->vm_mm;
> +
> +			if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> +				/*
> +				 * No need to hold VMA lock for migration.
> +				 * WARNING: vma can't be used after this!
> +				 */
> +				vma_end_read(vma);
> +				ret |= VM_FAULT_COMPLETED;
> +			}
> +			migration_entry_wait(mm, vmf->pmd, vmf->address);
>  		} else if (is_device_exclusive_entry(entry)) {
>  			vmf->page = pfn_swap_entry_to_page(entry);
>  			ret = remove_device_exclusive_entry(vmf);
> -- 
> 2.41.0.178.g377b9f9a00-goog
> 

-- 
Peter Xu