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

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Date:   Mon, 8 Nov 2021 09:23:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Lang Yu <Lang.Yu@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/kmemleak: Avoid scanning potential huge holes

On 08.11.21 08:27, Lang Yu wrote:
> On Fri, Nov 05, 2021 at 02:14:50PM +0100, David Hildenbrand wrote:
>> 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., 0x400000000 ,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;
> 
> The original code has covered that, i.e., 
> only scan pages belonging to this node.
> I didn't change that behavior.

Again, you can easily have overlapping zones -- ZONE_NORMAL and
ZONE_MOVABLE -- in which case, a PFN is spanned by multiple zones, but
only belongs to a single zone.

The original code would scan each PFN exactly once, as it was iterating
the node PFNs. Your changed code might scan a single PFN multiple times,
if it's spanned by multiple zones.

-- 
Thanks,

David / dhildenb