linux-kernel - Re: [PATCH] swap: cleanup get/put_swap

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Message-ID: <d1b054d0-d083-d35c-e547-7e8756fd802a@redhat.com>
Date:   Tue, 16 May 2023 13:06:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Huang Ying <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Minchan Kim <minchan@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH] swap: cleanup get/put_swap_device usage

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