[PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Mike Kravetz]

If the call __alloc_contig_migrate_range() in alloc_contig_range
returns -EBUSY, processing continues so that test_pages_isolated()
is called where there is a tracepoint to identify the busy pages.
However, it is possible for busy pages to become available between
the calls to these two routines.  In this case, the range of pages
may be allocated.   Unfortunately, the original return code (ret
== -EBUSY) is still set and returned to the caller.  Therefore,
the caller believes the pages were not allocated and they are leaked.

Update the return code with the value from test_pages_isolated().

Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
Cc: <stable@vger.kernel.org>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/page_alloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 77e4d3c5c57b..3605ca82fd29 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 	}
 
 	/* Make sure the range is really isolated. */
-	if (test_pages_isolated(outer_start, end, false)) {
+	ret = test_pages_isolated(outer_start, end, false);
+	if (ret) {
 		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
 			__func__, outer_start, end);
-		ret = -EBUSY;
 		goto done;
 	}
 
-- 
2.13.6

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Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Vlastimil Babka]

On 11/20/2017 08:39 PM, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update the return code with the value from test_pages_isolated().

Good catch and seems ok for a stable fix. But it's another indication
that this area needs some larger rewrite.

For example, it seems that the tracepoints in test_pages_isolated() will
report not only pages which were busy during migration attempt, but also
pages that were not at all attempted, because
__alloc_contig_migrate_range() gave up?

> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c5c57b..3605ca82fd29 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	}
>  
>  	/* Make sure the range is really isolated. */
> -	if (test_pages_isolated(outer_start, end, false)) {
> +	ret = test_pages_isolated(outer_start, end, false);
> +	if (ret) {
>  		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
>  			__func__, outer_start, end);
> -		ret = -EBUSY;
>  		goto done;
>  	}
>  
> 

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Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Michal Hocko]

On Tue 21-11-17 08:53:11, Vlastimil Babka wrote:
> On 11/20/2017 08:39 PM, Mike Kravetz wrote:
> > If the call __alloc_contig_migrate_range() in alloc_contig_range
> > returns -EBUSY, processing continues so that test_pages_isolated()
> > is called where there is a tracepoint to identify the busy pages.
> > However, it is possible for busy pages to become available between
> > the calls to these two routines.  In this case, the range of pages
> > may be allocated.   Unfortunately, the original return code (ret
> > == -EBUSY) is still set and returned to the caller.  Therefore,
> > the caller believes the pages were not allocated and they are leaked.
> > 
> > Update the return code with the value from test_pages_isolated().
> 
> Good catch and seems ok for a stable fix. But it's another indication
> that this area needs some larger rewrite.

Absolutely. The whole thing is subtle as hell. And shaping the code just
around the tracepoint here smells like the whole design could be thought
through much more.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Michal Hocko]

On Mon 20-11-17 11:39:30, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update the return code with the value from test_pages_isolated().

I find the description in 0/0 much more easier to grasp so I would
vote for merging it into the changelog. It took me a while to realize
that we are talking about the success path just from staring into the
diff which doesn't have a sufficient scope to tell the whole story.

Btw. I find embeded description in the diffstat area much more easier to
follow for single patches. Having a cover for a single patch sounds more
like a distraction. Just my 2c

> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/page_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c5c57b..3605ca82fd29 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	}
>  
>  	/* Make sure the range is really isolated. */
> -	if (test_pages_isolated(outer_start, end, false)) {
> +	ret = test_pages_isolated(outer_start, end, false);
> +	if (ret) {
>  		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
>  			__func__, outer_start, end);
> -		ret = -EBUSY;
>  		goto done;
>  	}
>  
> -- 
> 2.13.6
> 

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Michal Nazarewicz]

On Mon, Nov 20 2017, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
>
> Update the return code with the value from test_pages_isolated().
>
> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Acked-by: Michal Nazarewicz <mina86@mina86.com>

> ---
>  mm/page_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c5c57b..3605ca82fd29 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	}
>  
>  	/* Make sure the range is really isolated. */
> -	if (test_pages_isolated(outer_start, end, false)) {
> +	ret = test_pages_isolated(outer_start, end, false);
> +	if (ret) {
>  		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
>  			__func__, outer_start, end);
> -		ret = -EBUSY;
>  		goto done;
>  	}

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»

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Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Johannes Weiner]

On Mon, Nov 20, 2017 at 11:39:30AM -0800, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update the return code with the value from test_pages_isolated().
> 
> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Wow, good catch.

> ---
>  mm/page_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c5c57b..3605ca82fd29 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	}
>  
>  	/* Make sure the range is really isolated. */
> -	if (test_pages_isolated(outer_start, end, false)) {
> +	ret = test_pages_isolated(outer_start, end, false);
> +	if (ret) {
>  		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
>  			__func__, outer_start, end);
> -		ret = -EBUSY;
>  		goto done;

Essentially, an -EBUSY from __alloc_contig_migrate_range() doesn't
mean anything, and we return 0 if the rest of the operations succeed.

Since we never plan on returning that particular -EBUSY, would it be
more robust to reset it right then and there, rather than letting it
run on in ret for more than a screenful?

It would also be good to note in that fall-through comment that the
pages becoming free on their own is a distinct possibility.

As Michal points out, this is really subtle. It makes sense to make it
as explicit as possible.

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Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential leak

[Mike Kravetz]

On 11/22/2017 04:00 AM, Johannes Weiner wrote:
> On Mon, Nov 20, 2017 at 11:39:30AM -0800, Mike Kravetz wrote:
>> If the call __alloc_contig_migrate_range() in alloc_contig_range
>> returns -EBUSY, processing continues so that test_pages_isolated()
>> is called where there is a tracepoint to identify the busy pages.
>> However, it is possible for busy pages to become available between
>> the calls to these two routines.  In this case, the range of pages
>> may be allocated.   Unfortunately, the original return code (ret
>> == -EBUSY) is still set and returned to the caller.  Therefore,
>> the caller believes the pages were not allocated and they are leaked.
>>
>> Update the return code with the value from test_pages_isolated().
>>
>> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> Wow, good catch.
> 
>> ---
>>  mm/page_alloc.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 77e4d3c5c57b..3605ca82fd29 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>>  	}
>>  
>>  	/* Make sure the range is really isolated. */
>> -	if (test_pages_isolated(outer_start, end, false)) {
>> +	ret = test_pages_isolated(outer_start, end, false);
>> +	if (ret) {
>>  		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
>>  			__func__, outer_start, end);
>> -		ret = -EBUSY;
>>  		goto done;
> 
> Essentially, an -EBUSY from __alloc_contig_migrate_range() doesn't
> mean anything, and we return 0 if the rest of the operations succeed.
> 
> Since we never plan on returning that particular -EBUSY, would it be
> more robust to reset it right then and there, rather than letting it
> run on in ret for more than a screenful?
> 
> It would also be good to note in that fall-through comment that the
> pages becoming free on their own is a distinct possibility.
> 
> As Michal points out, this is really subtle. It makes sense to make it
> as explicit as possible.

Ok, I thought about zero'ing ret right after the call to
__alloc_contig_migrate_range and return of -EBUSY.  It just didn't look
right to me.  But, you are correct.  We should make this as explicit as
possible.  I will respin the patch as suggested and be sure to include an
explicit comment when setting ret = 0.

-- 
Mike Kravetz

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[PATCH v2] mm/cma: fix alloc_contig_range ret code/potential leak

[Mike Kravetz]

If the call __alloc_contig_migrate_range() in alloc_contig_range
returns -EBUSY, processing continues so that test_pages_isolated()
is called where there is a tracepoint to identify the busy pages.
However, it is possible for busy pages to become available between
the calls to these two routines.  In this case, the range of pages
may be allocated.   Unfortunately, the original return code (ret
== -EBUSY) is still set and returned to the caller.  Therefore,
the caller believes the pages were not allocated and they are leaked.

Update comment to indicate that allocation is still possible even if
__alloc_contig_migrate_range returns -EBUSY.  Also, clear return code
in this case so that it is not accidentally used or returned to caller.

Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
Cc: <stable@vger.kernel.org>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/page_alloc.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 77e4d3c5c57b..25e81844d1aa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7582,11 +7582,18 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 
 	/*
 	 * In case of -EBUSY, we'd like to know which page causes problem.
-	 * So, just fall through. We will check it in test_pages_isolated().
+	 * So, just fall through. test_pages_isolated() has a tracepoint
+	 * which will report the busy page.
+	 *
+	 * It is possible that busy pages could become available before
+	 * the call to test_pages_isolated, and the range will actually be
+	 * allocated.  So, if we fall through be sure to clear ret so that
+	 * -EBUSY is not accidentally used or returned to caller.
 	 */
 	ret = __alloc_contig_migrate_range(&cc, start, end);
 	if (ret && ret != -EBUSY)
 		goto done;
+	ret =0;
 
 	/*
 	 * Pages from [start, end) are within a MAX_ORDER_NR_PAGES
-- 
2.13.6

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Re: [PATCH v2] mm/cma: fix alloc_contig_range ret code/potential leak

[Vlastimil Babka]

On 11/22/2017 07:52 PM, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update comment to indicate that allocation is still possible even if
> __alloc_contig_migrate_range returns -EBUSY.  Also, clear return code
> in this case so that it is not accidentally used or returned to caller.
> 
> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c5c57b..25e81844d1aa 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7582,11 +7582,18 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  
>  	/*
>  	 * In case of -EBUSY, we'd like to know which page causes problem.
> -	 * So, just fall through. We will check it in test_pages_isolated().
> +	 * So, just fall through. test_pages_isolated() has a tracepoint
> +	 * which will report the busy page.
> +	 *
> +	 * It is possible that busy pages could become available before
> +	 * the call to test_pages_isolated, and the range will actually be
> +	 * allocated.  So, if we fall through be sure to clear ret so that
> +	 * -EBUSY is not accidentally used or returned to caller.
>  	 */
>  	ret = __alloc_contig_migrate_range(&cc, start, end);
>  	if (ret && ret != -EBUSY)
>  		goto done;
> +	ret =0;

             ^ missing space
>  
>  	/*
>  	 * Pages from [start, end) are within a MAX_ORDER_NR_PAGES
> 

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Re: [PATCH v2] mm/cma: fix alloc_contig_range ret code/potential leak

[Michal Hocko]

On Wed 22-11-17 10:52:14, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update comment to indicate that allocation is still possible even if
> __alloc_contig_migrate_range returns -EBUSY.  Also, clear return code
> in this case so that it is not accidentally used or returned to caller.
> 
> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

OK, this one looks reasonable as well.
Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/page_alloc.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c5c57b..25e81844d1aa 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7582,11 +7582,18 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  
>  	/*
>  	 * In case of -EBUSY, we'd like to know which page causes problem.
> -	 * So, just fall through. We will check it in test_pages_isolated().
> +	 * So, just fall through. test_pages_isolated() has a tracepoint
> +	 * which will report the busy page.
> +	 *
> +	 * It is possible that busy pages could become available before
> +	 * the call to test_pages_isolated, and the range will actually be
> +	 * allocated.  So, if we fall through be sure to clear ret so that
> +	 * -EBUSY is not accidentally used or returned to caller.
>  	 */
>  	ret = __alloc_contig_migrate_range(&cc, start, end);
>  	if (ret && ret != -EBUSY)
>  		goto done;
> +	ret =0;
>  
>  	/*
>  	 * Pages from [start, end) are within a MAX_ORDER_NR_PAGES
> -- 
> 2.13.6
> 

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH v2] mm/cma: fix alloc_contig_range ret code/potential leak

[Johannes Weiner]

On Wed, Nov 22, 2017 at 10:52:14AM -0800, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update comment to indicate that allocation is still possible even if
> __alloc_contig_migrate_range returns -EBUSY.  Also, clear return code
> in this case so that it is not accidentally used or returned to caller.
> 
> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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Re: [PATCH v2] mm/cma: fix alloc_contig_range ret code/potential leak

[Joonsoo Kim]

On Wed, Nov 22, 2017 at 10:52:14AM -0800, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update comment to indicate that allocation is still possible even if
> __alloc_contig_migrate_range returns -EBUSY.  Also, clear return code
> in this case so that it is not accidentally used or returned to caller.
> 
> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Good catch!!

Acked-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Thanks.

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