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[PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve_memtype

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  [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve_memtype Pan Xinhui <xinhuix.pan@intel.com> - 2015-07-22 07:50 +0200
    Re: [PATCH V3] x86/mm/pat: Do a small optimization and fix in  reserve_memtype Borislav Petkov <bp@alien8.de> - 2015-07-22 09:50 +0200
      Re: [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve_memtype Pan Xinhui <xinhuix.pan@intel.com> - 2015-07-22 15:00 +0200

#1189662 — [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve_memtype

From: Pan Xinhui <xinhuix.pan@intel.com>

It's more reasonable to unlock memtype_lock right after
rbt_memtype_check_insert. memtype_lock protects all data stored in
rb-tree from multiple access. It's not cool to call kfree, pr_info, etc
with this lock held. So move spin_unlock a little ahead.

If *new* succeed to be stored into the rb-tree, we might hit panic.
Because we access *new* in dprintk "cattr_name(new->type)". Data stored
in the rb-tree might be freed at any possbile time. It's abviously wrong
to access such data without lock held. As new->type might be changed in
rbt_memtype_check_insert, so save new->type to actual_type, then use
actual_type in dprintk.

Signed-off-by: Pan Xinhui <xinhuix.pan@intel.com>
---
change from v2:
	update comments.
change from V1:
	fix an access of *new* without memtype_lock held.
---
 arch/x86/mm/pat.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index 188e3e0..894a096 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -538,22 +538,25 @@ int reserve_memtype(u64 start, u64 end, enum page_cache_mode req_type,
 	new->type	= actual_type;
 
 	spin_lock(&memtype_lock);
-
 	err = rbt_memtype_check_insert(new, new_type);
+	/*
+	 * new->type might be changed in rbt_memtype_check_insert.
+	 * So save new->type to actual_type as dprintk uses it.
+	 * We are not allowed to touch new after unlocking memtype_lock.
+	 */
+	actual_type = new->type;
+	spin_unlock(&memtype_lock);
+
 	if (err) {
 		pr_info("x86/PAT: reserve_memtype failed [mem %#010Lx-%#010Lx], track %s, req %s\n",
 			start, end - 1,
 			cattr_name(new->type), cattr_name(req_type));
 		kfree(new);
-		spin_unlock(&memtype_lock);
-
 		return err;
 	}
 
-	spin_unlock(&memtype_lock);
-
 	dprintk("reserve_memtype added [mem %#010Lx-%#010Lx], track %s, req %s, ret %s\n",
-		start, end - 1, cattr_name(new->type), cattr_name(req_type),
+		start, end - 1, cattr_name(actual_type), cattr_name(req_type),
 		new_type ? cattr_name(*new_type) : "-");
 
 	return err;
-- 
1.9.1
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#1189713 — Re: [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve_memtype

On Wed, Jul 22, 2015 at 01:38:48PM +0800, Pan Xinhui wrote:
> From: Pan Xinhui <xinhuix.pan@intel.com>
> 
> It's more reasonable to unlock memtype_lock right after
> rbt_memtype_check_insert. memtype_lock protects all data stored in
> rb-tree from multiple access. It's not cool to call kfree, pr_info, etc
> with this lock held. So move spin_unlock a little ahead.
> 
> If *new* succeed to be stored into the rb-tree, we might hit panic.
> Because we access *new* in dprintk "cattr_name(new->type)". Data stored
> in the rb-tree might be freed at any possbile time. It's abviously wrong
> to access such data without lock held. As new->type might be changed in
> rbt_memtype_check_insert, so save new->type to actual_type, then use
> actual_type in dprintk.
> 
> Signed-off-by: Pan Xinhui <xinhuix.pan@intel.com>
> ---
> change from v2:
> 	update comments.
> change from V1:
> 	fix an access of *new* without memtype_lock held.
> ---
>  arch/x86/mm/pat.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)

This patch still doesn't update the comments over memtype_lock.

> 
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index 188e3e0..894a096 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -538,22 +538,25 @@ int reserve_memtype(u64 start, u64 end, enum page_cache_mode req_type,
>  	new->type	= actual_type;
>  
>  	spin_lock(&memtype_lock);
> -
>  	err = rbt_memtype_check_insert(new, new_type);
> +	/*
> +	 * new->type might be changed in rbt_memtype_check_insert.
> +	 * So save new->type to actual_type as dprintk uses it.
> +	 * We are not allowed to touch new after unlocking memtype_lock.
> +	 */
> +	actual_type = new->type;

We already assign actual_type to new->type above. I think the dprintk
needs actual_type and not what new->type has been changed to as that is
in new_type.

> +	spin_unlock(&memtype_lock);
> +
>  	if (err) {
>  		pr_info("x86/PAT: reserve_memtype failed [mem %#010Lx-%#010Lx], track %s, req %s\n",
>  			start, end - 1,
>  			cattr_name(new->type), cattr_name(req_type));
>  		kfree(new);
> -		spin_unlock(&memtype_lock);
> -
>  		return err;
>  	}
>  
> -	spin_unlock(&memtype_lock);
> -
>  	dprintk("reserve_memtype added [mem %#010Lx-%#010Lx], track %s, req %s, ret %s\n",
> -		start, end - 1, cattr_name(new->type), cattr_name(req_type),
> +		start, end - 1, cattr_name(actual_type), cattr_name(req_type),
>  		new_type ? cattr_name(*new_type) : "-");
>  
>  	return err;
> -- 
> 1.9.1

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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#1189877

hi, Borislav
	thanks for your reply. :)

On 2015年07月22日 18:46, Borislav Petkov wrote:
> On Wed, Jul 22, 2015 at 05:06:04PM +0800, Pan Xinhui wrote:
>> how about:
>> memtype_lock protects the rb-tree root and the rb-nodes which is a field of memtype from delete/add/lookup in a race.
> 
> Use this:
> 
> "All pat_rbtree operations need to be performed while holding the
> memtype_lock."
> 
thanks!

>> Actually I have same questions. I find these output logs are added in
>> commit: 6997ab4982a29925e79f72c3a59823cf944c3529(x86: add PAT related
>> debug prints) In the past, *new_type == actual_type == new->type on
>> success. codes are below. author use actual_type there.
> 
> So this function is one bit PITA. So req_type is used to compute actual
> type a bit higher:
> 
> 	actual_type = pat_x_mtrr_type(start, end, req_type);
> 
> and from then on actual_type is being used.
> 
> BUT!, in order to have *all* debugging information, the last dprintk()
agree, output all debugging information.

> call should dump actual_type and req_type because this way we show what
then why not append "act %s" to the dprintk format string?

> pat_x_mtrr_type() did too. And we don't need to dump new->type because
> this is the !err case and in that case we assigned new_type to it, which
> we dump already.
yes, new->type is same with *new_type, and dump same value twice.

> 
> Ok?
> 
Let me think for a while. I wonder why there is not any comment that could tell developers what "track %s" mean.
In different places of this file, "track %s" can mean what type of memory it is now, or it used to be.

So I think this output filed "track %s" is just whatever people want to need to print out.

I prefer to dump all debugging information here, so I agree with your idea. thanks

> Btw, you could also simplify this:
> 
> 	if (is_range_ram == 1) {
> 
> 		err = reserve_ram_pages_type(start, end, req_type, new_type);
> 
> 		return err;
> 	}
> 
> to:
> 
> 	if (is_range_ram == 1)
> 		return reserve_ram_pages_type(start, end, req_type, new_type);
> 
seems better now, thanks!
> while at it.
> 
> Thanks.
> 

thanks
xinhui
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