Re: [PATCH 4/5] swap: remove the swap lock in swap_cache_get_folio

[Huang, Ying]

Kairui Song <ryncsn@xxxxxxxxx> writes:

> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> There is only one caller not keep holding a reference or lock the
> swap device while calling this function. Just move the lock out
> of this function, it only used to prevent swapoff, and this helper
> function is very short so there is no performance regression
> issue. Help saves a few cycles.

> Subject: Re: [PATCH 4/5] swap: remove the swap lock in swap_cache_get_folio

I don't think you remove `swap lock` in swap_cache_get_folio().  Just
avoid to inc/dec the reference count.

And I think it's better to add '()' after swap_cache_get_folio to make
it clear it's a function.

> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/shmem.c      | 8 +++++++-
>  mm/swap_state.c | 8 ++------
>  2 files changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index c1d8b8a1aa3b..0183b6678270 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1725,6 +1725,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>  	struct address_space *mapping = inode->i_mapping;
>  	struct shmem_inode_info *info = SHMEM_I(inode);
>  	struct mm_struct *charge_mm = vma ? vma->vm_mm : NULL;
> +	struct swap_info_struct *si;
>  	struct folio *folio = NULL;
>  	swp_entry_t swap;
>  	int error;
> @@ -1737,7 +1738,12 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>  		return -EIO;
>  
>  	/* Look it up and read it in.. */
> -	folio = swap_cache_get_folio(swap, NULL, 0);
> +	si = get_swap_device(swap);
> +	if (si) {
> +		folio = swap_cache_get_folio(swap, NULL, 0);
> +		put_swap_device(si);

I'd rather to call put_swap_device() at the end of function.  That is,
whenever we get a swap entry without proper lock/reference to prevent
swapoff, we should call get_swap_device() to check its validity and
prevent the swap device from swapoff.

Best Regards,
Huang, Ying

> +	}
> +
>  	if (!folio) {
>  		/* Or update major stats only when swapin succeeds?? */
>  		if (fault_type) {
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 19089417abd1..eba388f67741 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -324,19 +324,15 @@ static inline bool swap_use_vma_readahead(void)
>   * unlocked and with its refcount incremented - we rely on the kernel
>   * lock getting page table operations atomic even if we drop the folio
>   * lock before returning.
> + *
> + * Caller must lock the swap device or hold a reference to keep it valid.
>   */
>  struct folio *swap_cache_get_folio(swp_entry_t entry,
>  		struct vm_area_struct *vma, unsigned long addr)
>  {
>  	struct folio *folio;
> -	struct swap_info_struct *si;
>  
> -	si = get_swap_device(entry);
> -	if (!si)
> -		return NULL;
>  	folio = filemap_get_folio(swap_address_space(entry), swp_offset(entry));
> -	put_swap_device(si);
> -
>  	if (folio) {
>  		bool vma_ra = swap_use_vma_readahead();
>  		bool readahead;