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

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Date:   Mon, 8 Nov 2021 15:27:44 +0800
From:   Lang Yu <Lang.Yu@....com>
To:     David Hildenbrand <david@...hat.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 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.

Thanks,
Lang

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