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

Peter Xu
2023-06-27T15:49:58Z

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