Re: shmat(2) returns page size aligned memory address

Re: shmat(2) returns page size aligned memory address

[Yubin Ruan]

Cc linux-mm

2017-10-08 23:37 GMT+08:00 Yubin Ruan <ablacktshirt@gmail.com>:
> Hi Michael,
> At the current man page for shmat(2)[1], there is no mentioning
> whether the returned memory address of shmat(2) will be page size
> aligned or not. As that is quite important to many applications(e.g.,
> those that use locks heavily and would like to avoid some locks by
> some atomic guarantees provided by the CPU), it would be great to
> specify that for Linux.
>
> I walked down the current implementation of shmat(2) in the latest
> kernel src and found that shmat(2) does return a page size aligned
> memory address:
>
> SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
>  -> do_shmat(...)
>  -> do_mmap_pgoff(...)
>  -> do_mmap(...)
>  -> get_unmapped_area(...)
>  -> get_area(...) -> offset_in_page(addr)
>
> there is a `offset_in_page(addr)' assertion at the end and if that is
> true a -EINVAL would be returned, by which we can be sure that
> shmat(2) will return a page size aligned memory address on success[2].
>
> I will create a patch later if that is acceptable.
>
> Thanks,
> Yubin
>
> [1]: http://man7.org/linux/man-pages/man2/shmat.2.html
> [2]: there is also a `offset_in_page(2)' in get_unmapped_area(...),
> but that doesn't lead to -EINVAL...I am not sure whether the logic of
> that code is right.

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[PATCH] shmat(2) returns page size aligned memory address

[Yubin Ruan]

On Sun, Oct 08, 2017 at 11:37:05PM +0800, Yubin Ruan wrote:
> Hi Michael,
> At the current man page for shmat(2)[1], there is no mentioning
> whether the returned memory address of shmat(2) will be page size
> aligned or not. As that is quite important to many applications(e.g.,
> those that use locks heavily and would like to avoid some locks by
> some atomic guarantees provided by the CPU), it would be great to
> specify that for Linux.
> 
> I walked down the current implementation of shmat(2) in the latest
> kernel src and found that shmat(2) does return a page size aligned
> memory address:
> 
> SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
>  -> do_shmat(...)
>  -> do_mmap_pgoff(...)
>  -> do_mmap(...)
>  -> get_unmapped_area(...)
>  -> get_area(...) -> offset_in_page(addr)
> 
> there is a `offset_in_page(addr)' assertion at the end and if that is
> true a -EINVAL would be returned, by which we can be sure that
> shmat(2) will return a page size aligned memory address on success[2].
> 
> I will create a patch later if that is acceptable.
> 
> Thanks,
> Yubin
> 
> [1]: http://man7.org/linux/man-pages/man2/shmat.2.html
> [2]: there is also a `offset_in_page(2)' in get_unmapped_area(...),
> but that doesn't lead to -EINVAL...I am not sure whether the logic of
> that code is right.

add the page-alignment attribute of the return address of shmat(2)
---

diff --git a/man2/shmop.2 b/man2/shmop.2
index 849529f..b8d7595 100644
--- a/man2/shmop.2
+++ b/man2/shmop.2
@@ -63,7 +63,7 @@ with one of the following criteria:
 If
 .I shmaddr
 is NULL,
-the system chooses a suitable (unused) address at which to attach
+the system chooses a suitable (unused) page-aligned address to attach
 the segment.
 .IP *
 If

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Re: [PATCH] shmat(2) returns page size aligned memory address

[Michael Kerrisk (man-opages)]

Thanks, Yubin. Patch applied.

Cheers,

Michael


On 10/09/2017 04:27 PM, Yubin Ruan wrote:
> On Sun, Oct 08, 2017 at 11:37:05PM +0800, Yubin Ruan wrote:
>> Hi Michael,
>> At the current man page for shmat(2)[1], there is no mentioning
>> whether the returned memory address of shmat(2) will be page size
>> aligned or not. As that is quite important to many applications(e.g.,
>> those that use locks heavily and would like to avoid some locks by
>> some atomic guarantees provided by the CPU), it would be great to
>> specify that for Linux.
>>
>> I walked down the current implementation of shmat(2) in the latest
>> kernel src and found that shmat(2) does return a page size aligned
>> memory address:
>>
>> SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
>>   -> do_shmat(...)
>>   -> do_mmap_pgoff(...)
>>   -> do_mmap(...)
>>   -> get_unmapped_area(...)
>>   -> get_area(...) -> offset_in_page(addr)
>>
>> there is a `offset_in_page(addr)' assertion at the end and if that is
>> true a -EINVAL would be returned, by which we can be sure that
>> shmat(2) will return a page size aligned memory address on success[2].
>>
>> I will create a patch later if that is acceptable.
>>
>> Thanks,
>> Yubin
>>
>> [1]: http://man7.org/linux/man-pages/man2/shmat.2.html
>> [2]: there is also a `offset_in_page(2)' in get_unmapped_area(...),
>> but that doesn't lead to -EINVAL...I am not sure whether the logic of
>> that code is right.
> 
> add the page-alignment attribute of the return address of shmat(2)
> ---
> 
> diff --git a/man2/shmop.2 b/man2/shmop.2
> index 849529f..b8d7595 100644
> --- a/man2/shmop.2
> +++ b/man2/shmop.2
> @@ -63,7 +63,7 @@ with one of the following criteria:
>   If
>   .I shmaddr
>   is NULL,
> -the system chooses a suitable (unused) address at which to attach
> +the system chooses a suitable (unused) page-aligned address to attach
>   the segment.
>   .IP *
>   If
> 

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