linux-kernel - Re: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap enabled

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Date:   Thu, 8 Feb 2018 02:17:52 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Huang Ying <huang.ying.caritas@...il.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Dan Streetman <ddstreet@...e.org>,
        Seth Jennings <sjenning@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Shaohua Li <shli@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Shakeel Butt <shakeelb@...gle.com>, stable@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap
 enabled

On Wed, Feb 07, 2018 at 03:00:35PM +0800, Huang, Ying wrote:
> From: Huang Ying <huang.ying.caritas@...il.com>
> 
> It was reported by Sergey Senozhatsky that if THP (Transparent Huge
> Page) and frontswap (via zswap) are both enabled, when memory goes low
> so that swap is triggered, segfault and memory corruption will occur
> in random user space applications as follow,
> 
> kernel: urxvt[338]: segfault at 20 ip 00007fc08889ae0d sp 00007ffc73a7fc40 error 6 in libc-2.26.so[7fc08881a000+1ae000]
>  #0  0x00007fc08889ae0d _int_malloc (libc.so.6)
>  #1  0x00007fc08889c2f3 malloc (libc.so.6)
>  #2  0x0000560e6004bff7 _Z14rxvt_wcstoutf8PKwi (urxvt)
>  #3  0x0000560e6005e75c n/a (urxvt)
>  #4  0x0000560e6007d9f1 _ZN16rxvt_perl_interp6invokeEP9rxvt_term9hook_typez (urxvt)
>  #5  0x0000560e6003d988 _ZN9rxvt_term9cmd_parseEv (urxvt)
>  #6  0x0000560e60042804 _ZN9rxvt_term6pty_cbERN2ev2ioEi (urxvt)
>  #7  0x0000560e6005c10f _Z17ev_invoke_pendingv (urxvt)
>  #8  0x0000560e6005cb55 ev_run (urxvt)
>  #9  0x0000560e6003b9b9 main (urxvt)
>  #10 0x00007fc08883af4a __libc_start_main (libc.so.6)
>  #11 0x0000560e6003f9da _start (urxvt)
> 
> After bisection, it was found the first bad commit is
> bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped
> out").
> 
> The root cause is as follow.
> 
> When the pages are written to swap device during swapping out in
> swap_writepage(), zswap (fontswap) is tried to compress the pages
> instead to improve the performance.  But zswap (frontswap) will treat
> THP as normal page, so only the head page is saved.  After swapping
> in, tail pages will not be restored to its original contents, so cause
> the memory corruption in the applications.
> 
> This is fixed via splitting THP before writing the page to swap device
> if frontswap is enabled.  To deal with the situation where frontswap
> is enabled at runtime, whether the page is THP is checked before using
> frontswap during swapping out too.
> 
> Reported-and-tested-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: Dan Streetman <ddstreet@...e.org>
> Cc: Seth Jennings <sjenning@...hat.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Cc: Shaohua Li <shli@...nel.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Shakeel Butt <shakeelb@...gle.com>
> Cc: stable@...r.kernel.org # 4.14
> Fixes: bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped out")
> 
> Changelog:
> 
> v2:
> 
> - Move frontswap check into swapfile.c to avoid to make vmscan.c
>   depends on frontswap.
> ---
>  mm/page_io.c  | 2 +-
>  mm/swapfile.c | 3 +++
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_io.c b/mm/page_io.c
> index b41cf9644585..6dca817ae7a0 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -250,7 +250,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
>  		unlock_page(page);
>  		goto out;
>  	}
> -	if (frontswap_store(page) == 0) {
> +	if (!PageTransHuge(page) && frontswap_store(page) == 0) {

Why do we need this?

If frontswap_enabled is enabled but it doesn't support THP, it doesn't allow
cluster allocation by below logic so any THP page shouldn't come this path.
What do I missing now?

>  		set_page_writeback(page);
>  		unlock_page(page);
>  		end_page_writeback(page);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 006047b16814..0b7c7883ce64 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -934,6 +934,9 @@ int get_swap_pages(int n_goal, bool cluster, swp_entry_t swp_entries[])
>  
>  	/* Only single cluster request supported */
>  	WARN_ON_ONCE(n_goal > 1 && cluster);
> +	/* Frontswap doesn't support THP */
> +	if (frontswap_enabled() && cluster)
> +		goto noswap;
>  
>  	avail_pgs = atomic_long_read(&nr_swap_pages) / nr_pages;
>  	if (avail_pgs <= 0)
> -- 
> 2.15.1
>