Re: [PATCH v4 5/6] mm: handle swap page faults under per-VMA lock

[Peter Xu]

On Wed, Jun 28, 2023 at 12:17:59AM -0700, Suren Baghdasaryan wrote:
> When page fault is handled under per-VMA lock protection, all swap page
> faults are retried with mmap_lock because folio_lock_or_retry has to drop
> and reacquire mmap_lock if folio could not be immediately locked.
> Follow the same pattern as mmap_lock to drop per-VMA lock when waiting
> for folio and retrying once folio is available.
> With this obstacle removed, enable do_swap_page to operate under
> per-VMA lock protection. Drivers implementing ops->migrate_to_ram might
> still rely on mmap_lock, therefore we have to fall back to mmap_lock in
> that particular case.
> Note that the only time do_swap_page calls synchronous swap_readpage
> is when SWP_SYNCHRONOUS_IO is set, which is only set for
> QUEUE_FLAG_SYNCHRONOUS devices: brd, zram and nvdimms (both btt and
> pmem). Therefore we don't sleep in this path, and there's no need to
> drop the mmap or per-VMA lock.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Acked-by: Peter Xu <peterx@xxxxxxxxxx>

One nit below:

> ---
>  mm/filemap.c | 25 ++++++++++++++++---------
>  mm/memory.c  | 16 ++++++++++------
>  2 files changed, 26 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 52bcf12dcdbf..7ee078e1a0d2 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1699,31 +1699,38 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
>  	return ret;
>  }
>  
> +static void release_fault_lock(struct vm_fault *vmf)
> +{
> +	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> +		vma_end_read(vmf->vma);
> +	else
> +		mmap_read_unlock(vmf->vma->vm_mm);
> +}
> +
>  /*
>   * Return values:
>   * 0 - folio is locked.
>   * VM_FAULT_RETRY - folio is not locked.
> - *     mmap_lock has been released (mmap_read_unlock(), unless flags had both
> - *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
> - *     which case mmap_lock is still held.
> + *     mmap_lock or per-VMA lock has been released (mmap_read_unlock() or
> + *     vma_end_read()), unless flags had both FAULT_FLAG_ALLOW_RETRY and
> + *     FAULT_FLAG_RETRY_NOWAIT set, in which case the lock is still held.
>   *
>   * If neither ALLOW_RETRY nor KILLABLE are set, will always return 0
> - * with the folio locked and the mmap_lock unperturbed.
> + * with the folio locked and the mmap_lock/per-VMA lock is left unperturbed.
>   */
>  vm_fault_t __folio_lock_or_retry(struct folio *folio, struct vm_fault *vmf)
>  {
> -	struct mm_struct *mm = vmf->vma->vm_mm;
>  	unsigned int flags = vmf->flags;
>  
>  	if (fault_flag_allow_retry_first(flags)) {
>  		/*
> -		 * CAUTION! In this case, mmap_lock is not released
> -		 * even though return VM_FAULT_RETRY.
> +		 * CAUTION! In this case, mmap_lock/per-VMA lock is not
> +		 * released even though returning VM_FAULT_RETRY.
>  		 */
>  		if (flags & FAULT_FLAG_RETRY_NOWAIT)
>  			return VM_FAULT_RETRY;
>  
> -		mmap_read_unlock(mm);
> +		release_fault_lock(vmf);
>  		if (flags & FAULT_FLAG_KILLABLE)
>  			folio_wait_locked_killable(folio);
>  		else
> @@ -1735,7 +1742,7 @@ vm_fault_t __folio_lock_or_retry(struct folio *folio, struct vm_fault *vmf)
>  
>  		ret = __folio_lock_killable(folio);
>  		if (ret) {
> -			mmap_read_unlock(mm);
> +			release_fault_lock(vmf);
>  			return VM_FAULT_RETRY;
>  		}
>  	} else {
> diff --git a/mm/memory.c b/mm/memory.c
> index 345080052003..76c7907e7286 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3712,12 +3712,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	if (!pte_unmap_same(vmf))
>  		goto out;
>  
> -	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> -		ret = VM_FAULT_RETRY;
> -		vma_end_read(vma);
> -		goto out;
> -	}
> -
>  	entry = pte_to_swp_entry(vmf->orig_pte);
>  	if (unlikely(non_swap_entry(entry))) {
>  		if (is_migration_entry(entry)) {
> @@ -3727,6 +3721,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  			vmf->page = pfn_swap_entry_to_page(entry);
>  			ret = remove_device_exclusive_entry(vmf);
>  		} else if (is_device_private_entry(entry)) {
> +			if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> +				/*
> +				 * migrate_to_ram is not yet ready to operate
> +				 * under VMA lock.
> +				 */
> +				vma_end_read(vma);
> +				ret |= VM_FAULT_RETRY;

Here IIUC ret==0 is guaranteed, so maybe "ret = VM_FAULT_RETRY" is slightly
clearer.

> +				goto out;
> +			}
> +
>  			vmf->page = pfn_swap_entry_to_page(entry);
>  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>  					vmf->address, &vmf->ptl);
> -- 
> 2.41.0.162.gfafddb0af9-goog
> 

-- 
Peter Xu