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

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Message-ID: <87fs7v7qmh.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Wed, 17 May 2023 08:23:18 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, 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

David Hildenbrand <david@...hat.com> writes:

> 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.

OK.  Will do that in the next version.

Best Regards,
Huang, Ying