linux-kernel - Re: [PATCH v2] mm/kmemleak: Avoid scanning potential huge holes

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Date:   Wed, 24 Nov 2021 18:31:01 +0800
From:   Lang Yu <Lang.Yu@....com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, Oscar Salvador <osalvador@...e.de>,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] mm/kmemleak: Avoid scanning potential huge holes

On Wed, Nov 24, 2021 at 10:07:57AM +0100, David Hildenbrand wrote:
> On 08.11.21 15:00, Lang Yu wrote:
> > When using devm_request_free_mem_region() and devm_memremap_pages()
> > to add ZONE_DEVICE memory, if requested free mem region's end pfn
> > were huge(e.g., 0x400000000), the node_end_pfn() will be also huge
> > (see move_pfn_range_to_zone()). Thus it creates a huge hole between
> > node_start_pfn() and node_end_pfn().
> > 
> > We found on some AMD APUs, amdkfd requested such a free mem region
> > and created a huge hole. In such a case, following code snippet 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
> > 
> > v2:
> > - Only scan pages belonging to the zone.(David Hildenbrand)
> > - Use __maybe_unused to make compilers happy.
> > 
> > Signed-off-by: Lang Yu <lang.yu@....com>
> > ---
> >  mm/kmemleak.c | 13 +++++++------
> >  1 file changed, 7 insertions(+), 6 deletions(-)
> > 
> > diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> > index b57383c17cf6..adbe5aa01184 100644
> > --- a/mm/kmemleak.c
> > +++ b/mm/kmemleak.c
> > @@ -1403,7 +1403,8 @@ static void kmemleak_scan(void)
> >  {
> >  	unsigned long flags;
> >  	struct kmemleak_object *object;
> > -	int i;
> > +	struct zone *zone;
> > +	int __maybe_unused i;
> >  	int new_leaks = 0;
> >  
> >  	jiffies_last_scan = jiffies;
> > @@ -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++) {
> > @@ -1454,8 +1455,8 @@ static void kmemleak_scan(void)
> >  			if (!page)
> >  				continue;
> >  
> > -			/* only scan pages belonging to this node */
> > -			if (page_to_nid(page) != i)
> > +			/* only scan pages belonging to this zone */
> > +			if (page_zone(page) != zone)
> >  				continue;
> >  			/* only scan if page is in use */
> >  			if (page_count(page) == 0)
> > 
> 
> I think in theory we could optimize further, there really isn't that
> much need to skip single pages ... we can usually skip whole 
> pageblocks. (in some corner cases we might have to back off 
> one pageblock and continue the search page-wise). But that's a
> different story and there might not be need to optimize.

 I agree with you.

> 
> Also, I wonder if we should adjust the cond_resched() logic instead.
> While your code makes the "sparse node" case faster, I think we could
> still run into the same issue in the "sparse zone" case now.
> 
> Acked-by: David Hildenbrand <david@...hat.com>
> 
> to this patch.
> 
> 
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index b57383c17cf6..1cd1df3cb01b 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -1451,6 +1451,9 @@ static void kmemleak_scan(void)
>                 for (pfn = start_pfn; pfn < end_pfn; pfn++) {
>                         struct page *page = pfn_to_online_page(pfn);
>  
> +                       if (!(pfn & 63))
> +                               cond_resched();
> +
>                         if (!page)
>                                 continue;
>  
> @@ -1461,8 +1464,6 @@ static void kmemleak_scan(void)
>                         if (page_count(page) == 0)
>                                 continue;
>                         scan_block(page, page + 1, NULL);
> -                       if (!(pfn & 63))
> -                               cond_resched();
>                 }
>         }
>         put_online_mems();
> 
> 
> What do you think?

Yes, I think that will avoid any potential soft lockup.
But wheather there are still such huge continuous pages.
And the run time may increase a little.

Regards,
Lang

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