linux-kernel - Re: [PATCH] mm, swap: Remove WARN_ON_ONCE() in free_swap

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Date:   Tue, 21 Mar 2017 16:42:18 -0400
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Shaohua Li <shli@...nel.org>, Rik van Riel <riel@...hat.com>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, swap: Remove WARN_ON_ONCE() in free_swap_slot()

On Mon, 2017-03-20 at 14:26 +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@...el.com>
> 
> Before commit 452b94b8c8c7 ("mm/swap: don't BUG_ON() due to
> uninitialized swap slot cache"), the following bug is reported,
> 
>   ------------[ cut here ]------------
>   kernel BUG at mm/swap_slots.c:270!
>   invalid opcode: 0000 [#1] SMP
>   CPU: 5 PID: 1745 Comm: (sd-pam) Not tainted 4.11.0-rc1-00243-g24c534bb161b #1
>   Hardware name: System manufacturer System Product Name/Z170-K, BIOS
> 1803 05/06/2016
>   RIP: 0010:free_swap_slot+0xba/0xd0
>   Call Trace:
>    swap_free+0x36/0x40
>    do_swap_page+0x360/0x6d0
>    __handle_mm_fault+0x880/0x1080
>    handle_mm_fault+0xd0/0x240
>    __do_page_fault+0x232/0x4d0
>    do_page_fault+0x20/0x70
>    page_fault+0x22/0x30
>   ---[ end trace aefc9ede53e0ab21 ]---
> 
> This is raised by the BUG_ON(!swap_slot_cache_initialized) in
> free_swap_slot().  This is incorrect, because even if the swap slots
> cache fails to be initialized, the swap should operate properly
> without the swap slots cache.  And the use_swap_slot_cache check later
> in the function will protect the uninitialized swap slots cache case.
> 
> In commit 452b94b8c8c7, the BUG_ON() is replaced by WARN_ON_ONCE().
> In the patch, the WARN_ON_ONCE() is removed too.
> 

This replaces my previous patch to replace the BUG_ON.

Acked-by: Tim Chen <tim.c.chen@...ux.intel.com>

> Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> ---
>  mm/swap_slots.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> index 7ebb23836f68..b1ccb58ad397 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -267,8 +267,6 @@ int free_swap_slot(swp_entry_t entry)
>  {
>  	struct swap_slots_cache *cache;
>  
> -	WARN_ON_ONCE(!swap_slot_cache_initialized);
> -
>  	cache = &get_cpu_var(swp_slots);
>  	if (use_swap_slot_cache && cache->slots_ret) {
>  		spin_lock_irq(&cache->free_lock);