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From:   Alistair Popple <apopple@nvidia.com>
To:     Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH v3 7/8] mm: drop VMA lock before waiting for migration
Date:   Tue, 27 Jun 2023 18:02:10 +1000
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Suren Baghdasaryan <surenb@google.com> writes:

> migration_entry_wait does not need VMA lock, therefore it can be
> dropped before waiting.
>
> 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;

Doesn't this need to also set FAULT_FLAG_LOCK_DROPPED to ensure we don't
call vma_end_read() again in __handle_mm_fault()?

> +			}
> +			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);