diff mbox series

mm/msync: exit early when the flags is an MS_ASYNC and start < vm_start

Message ID 20201012160911.6282-1-sh1r4s3@mail.si-head.nl (mailing list archive)
State New, archived
Headers show
Series mm/msync: exit early when the flags is an MS_ASYNC and start < vm_start | expand

Commit Message

Nikita Ermakov Oct. 12, 2020, 4:09 p.m. UTC 
Exit from the loop over the VMA in the case when the flags
contain only an MS_ASYNC and start < vm_start. In this case msync()
would return with -ENOMEM anyway so make it return early.

Signed-off-by: Nikita Ermakov <sh1r4s3@mail.si-head.nl>
---
 mm/msync.c | 2 ++
 1 file changed, 2 insertions(+)


base-commit: 6824a8a9b4861d7df7ee132a952bdf6f84a99cb8

Comments

Vlastimil Babka Oct. 20, 2020, 10:19 a.m. UTC | #1 
On 10/12/20 6:09 PM, Nikita Ermakov wrote:
> Exit from the loop over the VMA in the case when the flags
> contain only an MS_ASYNC and start < vm_start. In this case msync()
> would return with -ENOMEM anyway so make it return early.
> 
> Signed-off-by: Nikita Ermakov <sh1r4s3@mail.si-head.nl>

AFAICS it can still return -EBUSY if there's MS_INVALIDATE and a mlocked vma. 
This is all subtle and I don't think we should risk breaking something for this 
optimization.

> ---
>   mm/msync.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/msync.c b/mm/msync.c
> index 69c6d2029531..ed20c3621d4c 100644
> --- a/mm/msync.c
> +++ b/mm/msync.c
> @@ -69,6 +69,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
>   			goto out_unlock;
>   		/* Here start < vma->vm_end. */
>   		if (start < vma->vm_start) {
> +			if (flags == MS_ASYNC)
> +				goto out_unlock;
>   			start = vma->vm_start;
>   			if (start >= end)
>   				goto out_unlock;
> 
> base-commit: 6824a8a9b4861d7df7ee132a952bdf6f84a99cb8
>
Nikita Ermakov Oct. 20, 2020, 2:38 p.m. UTC | #2 
Hi!

> On 20 Oct 2020, at 13:19, Vlastimil Babka <vbabka@suse.cz> wrote:
> 
> On 10/12/20 6:09 PM, Nikita Ermakov wrote:
>> Exit from the loop over the VMA in the case when the flags
>> contain only an MS_ASYNC and start < vm_start. In this case msync()
>> would return with -ENOMEM anyway so make it return early.
>> Signed-off-by: Nikita Ermakov <sh1r4s3@mail.si-head.nl>
> 
> AFAICS it can still return -EBUSY if there's MS_INVALIDATE and a mlocked vma. This is all subtle and I don't think we should risk breaking something for this optimization.
> 
Yes, it could. But in this patch the optimization works only in the case if the flag is MS_ASYNC. If the flag is (MS_ASYNC | MS_INVALIDATE).

>> ---
>>  mm/msync.c | 2 ++
>>  1 file changed, 2 insertions(+)
>> diff --git a/mm/msync.c b/mm/msync.c
>> index 69c6d2029531..ed20c3621d4c 100644
>> --- a/mm/msync.c
>> +++ b/mm/msync.c
>> @@ -69,6 +69,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
>>  			goto out_unlock;
>>  		/* Here start < vma->vm_end. */
>>  		if (start < vma->vm_start) {
>> +			if (flags == MS_ASYNC)
>> +				goto out_unlock;
>>  			start = vma->vm_start;
>>  			if (start >= end)
>>  				goto out_unlock;
>> base-commit: 6824a8a9b4861d7df7ee132a952bdf6f84a99cb8
>
Vlastimil Babka Oct. 20, 2020, 3:03 p.m. UTC | #3 
On 10/20/20 4:38 PM, Nikita wrote:
> Hi!
> 
>> On 20 Oct 2020, at 13:19, Vlastimil Babka <vbabka@suse.cz> wrote:
>> 
>> On 10/12/20 6:09 PM, Nikita Ermakov wrote:
>>> Exit from the loop over the VMA in the case when the flags
>>> contain only an MS_ASYNC and start < vm_start. In this case msync()
>>> would return with -ENOMEM anyway so make it return early.
>>> Signed-off-by: Nikita Ermakov <sh1r4s3@mail.si-head.nl>
>> 
>> AFAICS it can still return -EBUSY if there's MS_INVALIDATE and a mlocked vma. This is all subtle and I don't think we should risk breaking something for this optimization.
>> 
> Yes, it could. But in this patch the optimization works only in the case if the flag is MS_ASYNC. If the flag is (MS_ASYNC | MS_INVALIDATE).

Ah, right, it compares "== MS_ASYNC" not "& MS_ASYNC" so it's correct. A comment 
would be nice, such as:

We found an unmapped range and with MS_ASYNC and no MS_INVALIDATE there's 
nothing to do and the result will always be -ENOMEM, so we can return immediately.

>>> ---
>>>  mm/msync.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>> diff --git a/mm/msync.c b/mm/msync.c
>>> index 69c6d2029531..ed20c3621d4c 100644
>>> --- a/mm/msync.c
>>> +++ b/mm/msync.c
>>> @@ -69,6 +69,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
>>>  			goto out_unlock;
>>>  		/* Here start < vma->vm_end. */
>>>  		if (start < vma->vm_start) {
>>> +			if (flags == MS_ASYNC)
>>> +				goto out_unlock;
>>>  			start = vma->vm_start;
>>>  			if (start >= end)
>>>  				goto out_unlock;
>>> base-commit: 6824a8a9b4861d7df7ee132a952bdf6f84a99cb8
>> 
>
Nikita Ermakov Oct. 20, 2020, 4:19 p.m. UTC | #4 
> On 20 Oct 2020, at 18:03, Vlastimil Babka <vbabka@suse.cz> wrote:
> 
> On 10/20/20 4:38 PM, Nikita wrote:
>> Hi!
>>> On 20 Oct 2020, at 13:19, Vlastimil Babka <vbabka@suse.cz> wrote:
>>> On 10/12/20 6:09 PM, Nikita Ermakov wrote:
>>>> Exit from the loop over the VMA in the case when the flags
>>>> contain only an MS_ASYNC and start < vm_start. In this case msync()
>>>> would return with -ENOMEM anyway so make it return early.
>>>> Signed-off-by: Nikita Ermakov <sh1r4s3@mail.si-head.nl>
>>> AFAICS it can still return -EBUSY if there's MS_INVALIDATE and a mlocked vma. This is all subtle and I don't think we should risk breaking something for this optimization.
>> Yes, it could. But in this patch the optimization works only in the case if the flag is MS_ASYNC. If the flag is (MS_ASYNC | MS_INVALIDATE).
> 
> Ah, right, it compares "== MS_ASYNC" not "& MS_ASYNC" so it's correct. A comment would be nice, such as:
> 
> We found an unmapped range and with MS_ASYNC and no MS_INVALIDATE there's nothing to do and the result will always be -ENOMEM, so we can return immediately.
> 
Thanks for the comment!
I should probably implement this comment to the patch description and resubmit it as v2.

>>>> ---
>>>> mm/msync.c | 2 ++
>>>> 1 file changed, 2 insertions(+)
>>>> diff --git a/mm/msync.c b/mm/msync.c
>>>> index 69c6d2029531..ed20c3621d4c 100644
>>>> --- a/mm/msync.c
>>>> +++ b/mm/msync.c
>>>> @@ -69,6 +69,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
>>>> 			goto out_unlock;
>>>> 		/* Here start < vma->vm_end. */
>>>> 		if (start < vma->vm_start) {
>>>> +			if (flags == MS_ASYNC)
>>>> +				goto out_unlock;
>>>> 			start = vma->vm_start;
>>>> 			if (start >= end)
>>>> 				goto out_unlock;
>>>> base-commit: 6824a8a9b4861d7df7ee132a952bdf6f84a99cb8
>
diff mbox series

Patch

diff --git a/mm/msync.c b/mm/msync.c
index 69c6d2029531..ed20c3621d4c 100644
--- a/mm/msync.c
+++ b/mm/msync.c
@@ -69,6 +69,8 @@  SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 			goto out_unlock;
 		/* Here start < vma->vm_end. */
 		if (start < vma->vm_start) {
+			if (flags == MS_ASYNC)
+				goto out_unlock;
 			start = vma->vm_start;
 			if (start >= end)
 				goto out_unlock;