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

[Minchan Kim]

Hi Huang,

On Tue, Feb 06, 2018 at 02:54:04PM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> 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 storage 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 at the begin of swapping out if
> frontswap is enabled.  To avoid frontswap to be enabled at runtime,
> whether the page is THP is checked before using frontswap during
> swapping out too.

Nice catch, Huang. However, before the adding a new dependency between
frontswap and vmscan that I want to avoid if it is possible, let's think
whether frontswap can support THP page or not.
Can't we handle it with some loop to handle all of subpages of THP page?
It might be not hard?

> 
> Reported-and-tested-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Dan Streetman <ddstreet@xxxxxxxx>
> Cc: Seth Jennings <sjenning@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 4.14
> Fixes: bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped out")
> ---
>  mm/page_io.c |  2 +-
>  mm/vmscan.c  | 16 +++++++++++++---
>  2 files changed, 14 insertions(+), 4 deletions(-)
> 
> 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) {
>  		set_page_writeback(page);
>  		unlock_page(page);
>  		end_page_writeback(page);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bee53495a829..d1c1e00b08bb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -55,6 +55,7 @@
>  
>  #include <linux/swapops.h>
>  #include <linux/balloon_compaction.h>
> +#include <linux/frontswap.h>
>  
>  #include "internal.h"
>  
> @@ -1063,14 +1064,23 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  					/* cannot split THP, skip it */
>  					if (!can_split_huge_page(page, NULL))
>  						goto activate_locked;
> +					/*
> +					 * Split THP if frontswap enabled,
> +					 * because it cannot process THP
> +					 */
> +					if (frontswap_enabled()) {
> +						if (split_huge_page_to_list(
> +							page, page_list))
> +							goto activate_locked;
> +					}
>  					/*
>  					 * Split pages without a PMD map right
>  					 * away. Chances are some or all of the
>  					 * tail pages can be freed without IO.
>  					 */
> -					if (!compound_mapcount(page) &&
> -					    split_huge_page_to_list(page,
> -								    page_list))
> +					else if (!compound_mapcount(page) &&
> +						 split_huge_page_to_list(page,
> +							page_list))
>  						goto activate_locked;
>  				}
>  				if (!add_to_swap(page)) {
> -- 
> 2.15.1
>