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

[Lang Yu <Lang.Yu@amd.com>]

On Mon, Nov 08, 2021 at 10:24:43AM +0100, David Hildenbrand wrote:
> On 08.11.21 10:06, Lang Yu wrote:
> > On Mon, Nov 08, 2021 at 09:23:16AM +0100, David Hildenbrand wrote:
> >> 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@amd.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.
> >>
> > 
> > Did you mean a single PFN is shared by multiple zones belonging to the 
> > same node here? Thanks!
> 
> Not shared, spanned. A PFN always belongs to exactly one ZONE+NODE, but
> might be "spanned" by multiple nodes or multiple zones, because nodes
> and zones can overlap We can get the actual zone of a PFN via
> page_zone(page) in my example above. Note that checking for the zone
> structure (not the zone number/idx) implicitly checks for the node.
> 
> 
> Let's take a look at an example:
> 
> ...
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory32/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory33/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory34/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory35/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory36/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory37/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory38/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory39/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory40/valid_zones
> Movable
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory41/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory42/valid_zones
> Movable
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory43/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory44/valid_zones
> Movable
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory45/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory46/valid_zones
> Movable
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory47/valid_zones
> Normal
> [root@vm-0 ~]# cat /sys/devices/system/memory/memory48/valid_zones
> 
> 
> # cat /proc/zoneinfo
> Node 0, zone      DMA
> 	...
>         spanned  4095
>         present  3998
>         managed  3977
> 	...
>   start_pfn:           1
> Node 0, zone    DMA32
> 	...
>         spanned  1044480
>         present  782304
>         managed  765920
> 	...
>   start_pfn:           4096
> Node 0, zone   Normal
> 	...
>         spanned  524288
>         present  393216
>         managed  365736
> 	...
>   start_pfn:           1048576
> Node 0, zone  Movable
> 	...
>         spanned  229376
>         present  131072
>         managed  131072
>   start_pfn:           1310720
> 
> 
> So Normal spans:
> 
> 1048576 -> 1572863
> 
> And Movable spans:
> 
> 1310720 -> 1540095
> 
> Both zones overlap.

Thanks for your clarification! I got it.

Regards,
Lang

> -- 
> Thanks,
> 
> David / dhildenb
>