Re: [PATCH v3 6/8] mm: handle swap page faults under per-VMA lock

[Peter Xu]

On Mon, Jun 26, 2023 at 09:23:19PM -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_fault (formerly
> known as folio_lock_or_retry) had 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 in folio_lock_fault 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>
> ---
>  mm/filemap.c | 24 ++++++++++++++++--------
>  mm/memory.c  | 21 ++++++++++++++-------
>  2 files changed, 30 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 8ad06d69895b..683f11f244cd 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1703,12 +1703,14 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
>   * 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.
> + *     FAULT_FLAG_LOCK_DROPPED bit in vmf flags will be set if mmap_lock or

This "FAULT_FLAG_LOCK_DROPPED" should belong to that patch when introduced.
But again I still think this flag as a whole with that patch is not needed
and should be dropped, unless I miss something important..

> + *     per-VMA lock got dropped. mmap_lock/per-VMA lock is dropped when
> + *     function fails to lock the folio, 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 unperturbed.
>   */
>  vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
>  {
> @@ -1716,13 +1718,16 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
>  
>  	if (fault_flag_allow_retry_first(vmf->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 (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
>  			return VM_FAULT_RETRY;
>  
> -		mmap_read_unlock(mm);
> +		if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> +			vma_end_read(vmf->vma);
> +		else
> +			mmap_read_unlock(mm);
>  		vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
>  		if (vmf->flags & FAULT_FLAG_KILLABLE)
>  			folio_wait_locked_killable(folio);
> @@ -1735,7 +1740,10 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
>  
>  		ret = __folio_lock_killable(folio);
>  		if (ret) {
> -			mmap_read_unlock(mm);
> +			if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> +				vma_end_read(vmf->vma);
> +			else
> +				mmap_read_unlock(mm);
>  			vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
>  			return VM_FAULT_RETRY;
>  		}
> diff --git a/mm/memory.c b/mm/memory.c
> index 3c2acafcd7b6..5caaa4c66ea2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3712,11 +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) {

So if with my imagination, here we'll already have the vma_read_end() and
this patch will remove it, which makes sense.  Then...

> -		ret = VM_FAULT_RETRY;
> -		goto out;
> -	}
> -
>  	entry = pte_to_swp_entry(vmf->orig_pte);
>  	if (unlikely(non_swap_entry(entry))) {
>  		if (is_migration_entry(entry)) {
> @@ -3726,6 +3721,15 @@ 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.
> +				 */

... here we probably just do vma_read_end(), then...

> +				ret |= VM_FAULT_RETRY;
> +				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);
> @@ -5089,9 +5093,12 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  		/*
>  		 * In case of VM_FAULT_RETRY or VM_FAULT_COMPLETED we might
>  		 * be still holding per-VMA lock to keep the vma stable as long
> -		 * as possible. Drop it before returning.
> +		 * as possible. In this situation vmf.flags has
> +		 * FAULT_FLAG_VMA_LOCK set and FAULT_FLAG_LOCK_DROPPED unset.
> +		 * Drop the lock before returning when this happens.
>  		 */
> -		if (vmf.flags & FAULT_FLAG_VMA_LOCK)
> +		if ((vmf.flags & (FAULT_FLAG_VMA_LOCK | FAULT_FLAG_LOCK_DROPPED)) ==
> +		    FAULT_FLAG_VMA_LOCK)
>  			vma_end_read(vma);

This whole chunk should have been dropped altogether with my comment in
previous patch, iiuc, and it should be no-op anyway for swap case.  For the
real "waiting for page lock during swapin" phase we should always 100%
release the vma lock in folio_lock_or_retry() - just like mmap lock.

Thanks,

>  	}
>  	return ret;
> -- 
> 2.41.0.178.g377b9f9a00-goog
> 

-- 
Peter Xu