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

[Andrew Morton]

On Wed,  7 Feb 2018 15:00:35 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:

> From: Huang Ying <huang.ying.caritas@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 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.
>
> ...
>
> --- 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/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;
>  

hm.  This is assuming that "cluster==true" means "this is thp swap". 
That's presently true, but is it appropriate that get_swap_pages() is
peeking at "cluster" to work out why it is being called?

Or would it be cleaner to do this in get_swap_page()?  Something like

--- a/mm/swap_slots.c~a
+++ a/mm/swap_slots.c
@@ -317,8 +317,11 @@ swp_entry_t get_swap_page(struct page *p
 	entry.val = 0;
 
 	if (PageTransHuge(page)) {
-		if (IS_ENABLED(CONFIG_THP_SWAP))
-			get_swap_pages(1, true, &entry);
+		/* Frontswap doesn't support THP */
+		if (!frontswap_enabled()) {
+			if (IS_ENABLED(CONFIG_THP_SWAP))
+				get_swap_pages(1, true, &entry);
+		}
 		return entry;
 	}
 
_