[v2,5/5] mm/shmem: fix shmem_swapin() race with swapoff
diff mbox series

Message ID 20210417094039.51711-6-linmiaohe@huawei.com
State New
Headers show
Series
  • close various race windows for swap
Related show

Commit Message

Miaohe Lin April 17, 2021, 9:40 a.m. UTC
When I was investigating the swap code, I found the below possible race
window:

CPU 1                                           CPU 2
-----                                           -----
shmem_swapin
  swap_cluster_readahead
    if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
                                                swapoff
                                                  si->flags &= ~SWP_VALID;
                                                  ..
                                                  synchronize_rcu();
                                                  ..
                                                  si->swap_file = NULL;
    struct inode *inode = si->swap_file->f_mapping->host;[oops!]

Close this race window by using get/put_swap_device() to guard against
concurrent swapoff.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/shmem.c | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Huang, Ying April 19, 2021, 2:15 a.m. UTC | #1
Miaohe Lin <linmiaohe@huawei.com> writes:

> When I was investigating the swap code, I found the below possible race
> window:
>
> CPU 1                                           CPU 2
> -----                                           -----
> shmem_swapin
>   swap_cluster_readahead
>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>                                                 swapoff
>                                                   si->flags &= ~SWP_VALID;
>                                                   ..
>                                                   synchronize_rcu();
>                                                   ..

You have removed these code in the previous patches of the series.  And
they are not relevant in this patch.

>                                                   si->swap_file = NULL;
>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>
> Close this race window by using get/put_swap_device() to guard against
> concurrent swapoff.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

No.  This isn't the commit that introduces the race condition.  Please
recheck your git blame result.

Best Regards,
Huang, Ying

> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/shmem.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..936ba5595297 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>  			struct shmem_inode_info *info, pgoff_t index)
>  {
> +	struct swap_info_struct *si;
>  	struct vm_area_struct pvma;
>  	struct page *page;
>  	struct vm_fault vmf = {
>  		.vma = &pvma,
>  	};
>  
> +	/* Prevent swapoff from happening to us. */
> +	si = get_swap_device(swap);
> +	if (unlikely(!si))
> +		return NULL;
>  	shmem_pseudo_vma_init(&pvma, info, index);
>  	page = swap_cluster_readahead(swap, gfp, &vmf);
>  	shmem_pseudo_vma_destroy(&pvma);
> +	put_swap_device(si);
>  
>  	return page;
>  }
Miaohe Lin April 19, 2021, 6:49 a.m. UTC | #2
On 2021/4/19 10:15, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> When I was investigating the swap code, I found the below possible race
>> window:
>>
>> CPU 1                                           CPU 2
>> -----                                           -----
>> shmem_swapin
>>   swap_cluster_readahead
>>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>>                                                 swapoff
>>                                                   si->flags &= ~SWP_VALID;
>>                                                   ..
>>                                                   synchronize_rcu();
>>                                                   ..
> 
> You have removed these code in the previous patches of the series.  And
> they are not relevant in this patch.

Yes, I should change these. Thanks.

> 
>>                                                   si->swap_file = NULL;
>>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>>
>> Close this race window by using get/put_swap_device() to guard against
>> concurrent swapoff.
>>
>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> 
> No.  This isn't the commit that introduces the race condition.  Please
> recheck your git blame result.
> 

I think this is really hard to find exact commit. I used git blame and found
this race should be existed when this is introduced. Any suggestion ?
Thanks.

> Best Regards,
> Huang, Ying
> 
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  mm/shmem.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 26c76b13ad23..936ba5595297 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>>  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>>  			struct shmem_inode_info *info, pgoff_t index)
>>  {
>> +	struct swap_info_struct *si;
>>  	struct vm_area_struct pvma;
>>  	struct page *page;
>>  	struct vm_fault vmf = {
>>  		.vma = &pvma,
>>  	};
>>  
>> +	/* Prevent swapoff from happening to us. */
>> +	si = get_swap_device(swap);
>> +	if (unlikely(!si))
>> +		return NULL;
>>  	shmem_pseudo_vma_init(&pvma, info, index);
>>  	page = swap_cluster_readahead(swap, gfp, &vmf);
>>  	shmem_pseudo_vma_destroy(&pvma);
>> +	put_swap_device(si);
>>  
>>  	return page;
>>  }
> .
>
Huang, Ying April 19, 2021, 7:04 a.m. UTC | #3
Miaohe Lin <linmiaohe@huawei.com> writes:

> On 2021/4/19 10:15, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@huawei.com> writes:
>> 
>>> When I was investigating the swap code, I found the below possible race
>>> window:
>>>
>>> CPU 1                                           CPU 2
>>> -----                                           -----
>>> shmem_swapin
>>>   swap_cluster_readahead
>>>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>>>                                                 swapoff
>>>                                                   si->flags &= ~SWP_VALID;
>>>                                                   ..
>>>                                                   synchronize_rcu();
>>>                                                   ..
>> 
>> You have removed these code in the previous patches of the series.  And
>> they are not relevant in this patch.
>
> Yes, I should change these. Thanks.
>
>> 
>>>                                                   si->swap_file = NULL;
>>>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>>>
>>> Close this race window by using get/put_swap_device() to guard against
>>> concurrent swapoff.
>>>
>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>> 
>> No.  This isn't the commit that introduces the race condition.  Please
>> recheck your git blame result.
>> 
>
> I think this is really hard to find exact commit. I used git blame and found
> this race should be existed when this is introduced. Any suggestion ?
> Thanks.

I think the commit that introduces the race condition is commit
8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or
not")

Best Regards,
Huang, Ying

>> Best Regards,
>> Huang, Ying
>> 
>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>> ---
>>>  mm/shmem.c | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>> index 26c76b13ad23..936ba5595297 100644
>>> --- a/mm/shmem.c
>>> +++ b/mm/shmem.c
>>> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>>>  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>>>  			struct shmem_inode_info *info, pgoff_t index)
>>>  {
>>> +	struct swap_info_struct *si;
>>>  	struct vm_area_struct pvma;
>>>  	struct page *page;
>>>  	struct vm_fault vmf = {
>>>  		.vma = &pvma,
>>>  	};
>>>  
>>> +	/* Prevent swapoff from happening to us. */
>>> +	si = get_swap_device(swap);
>>> +	if (unlikely(!si))
>>> +		return NULL;
>>>  	shmem_pseudo_vma_init(&pvma, info, index);
>>>  	page = swap_cluster_readahead(swap, gfp, &vmf);
>>>  	shmem_pseudo_vma_destroy(&pvma);
>>> +	put_swap_device(si);
>>>  
>>>  	return page;
>>>  }
>> .
>>
Miaohe Lin April 19, 2021, 7:14 a.m. UTC | #4
On 2021/4/19 15:04, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> On 2021/4/19 10:15, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>
>>>> When I was investigating the swap code, I found the below possible race
>>>> window:
>>>>
>>>> CPU 1                                           CPU 2
>>>> -----                                           -----
>>>> shmem_swapin
>>>>   swap_cluster_readahead
>>>>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>>>>                                                 swapoff
>>>>                                                   si->flags &= ~SWP_VALID;
>>>>                                                   ..
>>>>                                                   synchronize_rcu();
>>>>                                                   ..
>>>
>>> You have removed these code in the previous patches of the series.  And
>>> they are not relevant in this patch.
>>
>> Yes, I should change these. Thanks.
>>
>>>
>>>>                                                   si->swap_file = NULL;
>>>>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>>>>
>>>> Close this race window by using get/put_swap_device() to guard against
>>>> concurrent swapoff.
>>>>
>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>
>>> No.  This isn't the commit that introduces the race condition.  Please
>>> recheck your git blame result.
>>>
>>
>> I think this is really hard to find exact commit. I used git blame and found
>> this race should be existed when this is introduced. Any suggestion ?
>> Thanks.
> 
> I think the commit that introduces the race condition is commit
> 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or
> not")
> 

Thanks.
The commit log only describes one race condition. And for that one, this should be correct
Fixes tag. But there are still many other race conditions inside swap_cluster_readahead,
such as swap_readpage() called from swap_cluster_readahead. This tag could not cover the
all race windows.

> Best Regards,
> Huang, Ying
> 
>>> Best Regards,
>>> Huang, Ying
>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>> ---
>>>>  mm/shmem.c | 6 ++++++
>>>>  1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>> index 26c76b13ad23..936ba5595297 100644
>>>> --- a/mm/shmem.c
>>>> +++ b/mm/shmem.c
>>>> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>>>>  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>>>>  			struct shmem_inode_info *info, pgoff_t index)
>>>>  {
>>>> +	struct swap_info_struct *si;
>>>>  	struct vm_area_struct pvma;
>>>>  	struct page *page;
>>>>  	struct vm_fault vmf = {
>>>>  		.vma = &pvma,
>>>>  	};
>>>>  
>>>> +	/* Prevent swapoff from happening to us. */
>>>> +	si = get_swap_device(swap);
>>>> +	if (unlikely(!si))
>>>> +		return NULL;
>>>>  	shmem_pseudo_vma_init(&pvma, info, index);
>>>>  	page = swap_cluster_readahead(swap, gfp, &vmf);
>>>>  	shmem_pseudo_vma_destroy(&pvma);
>>>> +	put_swap_device(si);
>>>>  
>>>>  	return page;
>>>>  }
>>> .
>>>
> .
>
Huang, Ying April 19, 2021, 7:41 a.m. UTC | #5
Miaohe Lin <linmiaohe@huawei.com> writes:

> On 2021/4/19 15:04, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@huawei.com> writes:
>> 
>>> On 2021/4/19 10:15, Huang, Ying wrote:
>>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>>
>>>>> When I was investigating the swap code, I found the below possible race
>>>>> window:
>>>>>
>>>>> CPU 1                                           CPU 2
>>>>> -----                                           -----
>>>>> shmem_swapin
>>>>>   swap_cluster_readahead
>>>>>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>>>>>                                                 swapoff
>>>>>                                                   si->flags &= ~SWP_VALID;
>>>>>                                                   ..
>>>>>                                                   synchronize_rcu();
>>>>>                                                   ..
>>>>
>>>> You have removed these code in the previous patches of the series.  And
>>>> they are not relevant in this patch.
>>>
>>> Yes, I should change these. Thanks.
>>>
>>>>
>>>>>                                                   si->swap_file = NULL;
>>>>>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>>>>>
>>>>> Close this race window by using get/put_swap_device() to guard against
>>>>> concurrent swapoff.
>>>>>
>>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>>
>>>> No.  This isn't the commit that introduces the race condition.  Please
>>>> recheck your git blame result.
>>>>
>>>
>>> I think this is really hard to find exact commit. I used git blame and found
>>> this race should be existed when this is introduced. Any suggestion ?
>>> Thanks.
>> 
>> I think the commit that introduces the race condition is commit
>> 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or
>> not")
>> 
>
> Thanks.
> The commit log only describes one race condition. And for that one, this should be correct
> Fixes tag. But there are still many other race conditions inside swap_cluster_readahead,
> such as swap_readpage() called from swap_cluster_readahead. This tag could not cover the
> all race windows.

No. swap_readpage() in swap_cluster_readahead() is OK.  Because
__read_swap_cache_async() is called before that, so the swap entry will
be marked with SWAP_HAS_CACHE, and page will be locked.

Best Regards,
Huang, Ying

>> Best Regards,
>> Huang, Ying
>> 
>>>> Best Regards,
>>>> Huang, Ying
>>>>
>>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>>> ---
>>>>>  mm/shmem.c | 6 ++++++
>>>>>  1 file changed, 6 insertions(+)
>>>>>
>>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>>> index 26c76b13ad23..936ba5595297 100644
>>>>> --- a/mm/shmem.c
>>>>> +++ b/mm/shmem.c
>>>>> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>>>>>  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>>>>>  			struct shmem_inode_info *info, pgoff_t index)
>>>>>  {
>>>>> +	struct swap_info_struct *si;
>>>>>  	struct vm_area_struct pvma;
>>>>>  	struct page *page;
>>>>>  	struct vm_fault vmf = {
>>>>>  		.vma = &pvma,
>>>>>  	};
>>>>>  
>>>>> +	/* Prevent swapoff from happening to us. */
>>>>> +	si = get_swap_device(swap);
>>>>> +	if (unlikely(!si))
>>>>> +		return NULL;
>>>>>  	shmem_pseudo_vma_init(&pvma, info, index);
>>>>>  	page = swap_cluster_readahead(swap, gfp, &vmf);
>>>>>  	shmem_pseudo_vma_destroy(&pvma);
>>>>> +	put_swap_device(si);
>>>>>  
>>>>>  	return page;
>>>>>  }
>>>> .
>>>>
>> .
>>
Miaohe Lin April 19, 2021, 8:18 a.m. UTC | #6
On 2021/4/19 15:41, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> On 2021/4/19 15:04, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>
>>>> On 2021/4/19 10:15, Huang, Ying wrote:
>>>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>>>
>>>>>> When I was investigating the swap code, I found the below possible race
>>>>>> window:
>>>>>>
>>>>>> CPU 1                                           CPU 2
>>>>>> -----                                           -----
>>>>>> shmem_swapin
>>>>>>   swap_cluster_readahead
>>>>>>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>>>>>>                                                 swapoff
>>>>>>                                                   si->flags &= ~SWP_VALID;
>>>>>>                                                   ..
>>>>>>                                                   synchronize_rcu();
>>>>>>                                                   ..
>>>>>
>>>>> You have removed these code in the previous patches of the series.  And
>>>>> they are not relevant in this patch.
>>>>
>>>> Yes, I should change these. Thanks.
>>>>
>>>>>
>>>>>>                                                   si->swap_file = NULL;
>>>>>>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>>>>>>
>>>>>> Close this race window by using get/put_swap_device() to guard against
>>>>>> concurrent swapoff.
>>>>>>
>>>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>>>
>>>>> No.  This isn't the commit that introduces the race condition.  Please
>>>>> recheck your git blame result.
>>>>>
>>>>
>>>> I think this is really hard to find exact commit. I used git blame and found
>>>> this race should be existed when this is introduced. Any suggestion ?
>>>> Thanks.
>>>
>>> I think the commit that introduces the race condition is commit
>>> 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or
>>> not")
>>>
>>
>> Thanks.
>> The commit log only describes one race condition. And for that one, this should be correct
>> Fixes tag. But there are still many other race conditions inside swap_cluster_readahead,
>> such as swap_readpage() called from swap_cluster_readahead. This tag could not cover the
>> all race windows.
> 
> No. swap_readpage() in swap_cluster_readahead() is OK.  Because
> __read_swap_cache_async() is called before that, so the swap entry will
> be marked with SWAP_HAS_CACHE, and page will be locked.
> 

Oh... I missed this. Many thanks for your remind.

> Best Regards,
> Huang, Ying
> 
>>> Best Regards,
>>> Huang, Ying
>>>
>>>>> Best Regards,
>>>>> Huang, Ying
>>>>>
>>>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>>>> ---
>>>>>>  mm/shmem.c | 6 ++++++
>>>>>>  1 file changed, 6 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>>>> index 26c76b13ad23..936ba5595297 100644
>>>>>> --- a/mm/shmem.c
>>>>>> +++ b/mm/shmem.c
>>>>>> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>>>>>>  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>>>>>>  			struct shmem_inode_info *info, pgoff_t index)
>>>>>>  {
>>>>>> +	struct swap_info_struct *si;
>>>>>>  	struct vm_area_struct pvma;
>>>>>>  	struct page *page;
>>>>>>  	struct vm_fault vmf = {
>>>>>>  		.vma = &pvma,
>>>>>>  	};
>>>>>>  
>>>>>> +	/* Prevent swapoff from happening to us. */
>>>>>> +	si = get_swap_device(swap);
>>>>>> +	if (unlikely(!si))
>>>>>> +		return NULL;
>>>>>>  	shmem_pseudo_vma_init(&pvma, info, index);
>>>>>>  	page = swap_cluster_readahead(swap, gfp, &vmf);
>>>>>>  	shmem_pseudo_vma_destroy(&pvma);
>>>>>> +	put_swap_device(si);
>>>>>>  
>>>>>>  	return page;
>>>>>>  }
>>>>> .
>>>>>
>>> .
>>>
> .
>

Patch
diff mbox series

diff --git a/mm/shmem.c b/mm/shmem.c
index 26c76b13ad23..936ba5595297 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1492,15 +1492,21 @@  static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
 static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
 			struct shmem_inode_info *info, pgoff_t index)
 {
+	struct swap_info_struct *si;
 	struct vm_area_struct pvma;
 	struct page *page;
 	struct vm_fault vmf = {
 		.vma = &pvma,
 	};
 
+	/* Prevent swapoff from happening to us. */
+	si = get_swap_device(swap);
+	if (unlikely(!si))
+		return NULL;
 	shmem_pseudo_vma_init(&pvma, info, index);
 	page = swap_cluster_readahead(swap, gfp, &vmf);
 	shmem_pseudo_vma_destroy(&pvma);
+	put_swap_device(si);
 
 	return page;
 }