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

[Yubin Ruan <ablacktshirt@gmail.com>]

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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