Re: [PATCH] swap: cleanup get/put_swap_device usage

[David Hildenbrand]

On 16.05.23 07:29, Huang Ying wrote:
> The general rule to use a swap entry is as follows.
>
> When we get a swap entry, if there isn't some other way to prevent
> swapoff, such as page lock for swap cache, page table lock, etc., the
> swap entry may become invalid because of swapoff.  Then, we need to
> enclose all swap related functions with get_swap_device() and
> put_swap_device(), unless the swap functions call
> get/put_swap_device() by themselves.
>
> Add the rule as comments of get_swap_device(), and cleanup some
> functions which call get/put_swap_device().
>
> 1. Enlarge the get/put_swap_device() protection range in
> __read_swap_cache_async().  This makes the function a little easier to
> be understood because we don't need to consider swapoff.  And this
> makes it possible to remove get/put_swap_device() calling in some
> function called by __read_swap_cache_async().
>
> 2. Remove get/put_swap_device() in __swap_count().  Which is call in
> do_swap_page() only, which encloses the call with get/put_swap_device()
> already.
>
> 3. Remove get/put_swap_device() in __swp_swapcount().  Which is call
> in __read_swap_cache_async() only, which encloses the call with
> get/put_swap_device() already.
>
> 4. Remove get/put_swap_device() in __swap_duplicate(). Which is called
> by
>
> - swap_shmem_alloc(): the swap cache is locked.
>
> - copy_nonpresent_pte() -> swap_duplicate() and try_to_unmap_one() ->
> swap_duplicate(): the page table lock is held.
>
> - __read_swap_cache_async() -> swapcache_prepare(): enclosed with
> get/put_swap_device() already.
>
> Other get/put_swap_device() usages are checked too.

I suggest splitting this patch up into logical pieces as outlined here 
by you already.

--
Thanks,

David / dhildenb