linux-kernel - Re: [PATCH] mm/kmemleak: Avoid scanning potential huge holes

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Date:   Fri, 5 Nov 2021 14:14:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Lang Yu <lang.yu@....com>, linux-mm@...ck.org
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/kmemleak: Avoid scanning potential huge holes

On 05.11.21 04:52, Lang Yu wrote:
> When using devm_request_free_mem_region() and
> devm_memremap_pages() to add ZONE_DEVICE memory, if requested
> free mem region pfn were huge(e.g., 0x0x400000000 ,we found
> on some amd apus, amdkfd svm will request a such free mem region),
> the node_end_pfn() will be also huge(see move_pfn_range_to_zone()).
> It creates a huge hole between node_start_pfn() and node_end_pfn().
> 
> In such a case, following code snippet acctually was
> just doing busy test_bit() looping on the huge hole.
> 
> for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> 	struct page *page = pfn_to_online_page(pfn);
> 		if (!page)
> 			continue;
> 	...
> }
> 
> So we got a soft lockup:
> 
>  watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [bash:1221]
>  CPU: 6 PID: 1221 Comm: bash Not tainted 5.15.0-custom #1
>  RIP: 0010:pfn_to_online_page+0x5/0xd0
>  Call Trace:
>   ? kmemleak_scan+0x16a/0x440
>   kmemleak_write+0x306/0x3a0
>   ? common_file_perm+0x72/0x170
>   full_proxy_write+0x5c/0x90
>   vfs_write+0xb9/0x260
>   ksys_write+0x67/0xe0
>   __x64_sys_write+0x1a/0x20
>   do_syscall_64+0x3b/0xc0
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> I did some tests with the patch.
> 
> (1) amdgpu module unloaded
> 
> before the patch:
> 
> real    0m0.976s
> user    0m0.000s
> sys     0m0.968s
> 
> after the patch:
> 
> real    0m0.981s
> user    0m0.000s
> sys     0m0.973s
> 
> (2) amdgpu module loaded
> 
> before the patch:
> 
> real    0m35.365s
> user    0m0.000s
> sys     0m35.354s
> 
> after the patch:
> 
> real    0m1.049s
> user    0m0.000s
> sys     0m1.042s
> 
> Signed-off-by: Lang Yu <lang.yu@....com>
> ---
>  mm/kmemleak.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index b57383c17cf6..d07444613a84 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -1403,6 +1403,7 @@ static void kmemleak_scan(void)
>  {
>  	unsigned long flags;
>  	struct kmemleak_object *object;
> +	struct zone *zone;
>  	int i;
>  	int new_leaks = 0;
>  
> @@ -1443,9 +1444,9 @@ static void kmemleak_scan(void)
>  	 * Struct page scanning for each node.
>  	 */
>  	get_online_mems();
> -	for_each_online_node(i) {
> -		unsigned long start_pfn = node_start_pfn(i);
> -		unsigned long end_pfn = node_end_pfn(i);
> +	for_each_populated_zone(zone) {
> +		unsigned long start_pfn = zone->zone_start_pfn;
> +		unsigned long end_pfn = zone_end_pfn(zone);
>  		unsigned long pfn;
>  
>  		for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> @@ -1455,7 +1456,7 @@ static void kmemleak_scan(void)
>  				continue;
>  
>  			/* only scan pages belonging to this node */
> -			if (page_to_nid(page) != i)
> +			if (page_to_nid(page) != zone_to_nid(zone))

With overlapping zones you might rescan ranges ... instead we should do:

/* only scan pages belonging to this zone */
if (zone != page_zone(page))
	...

Or alternatively:

/* only scan pages belonging to this node */
if (page_to_nid(page) != zone_to_nid(zone))
	continue;
/* only scan pages belonging to this zone */
if (page_zonenum(page) != zone_idx(zone))
	continue;
-- 
Thanks,

David / dhildenb