From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael Kerrisk (man-opages)" Subject: Re: [PATCH] shmat(2) returns page size aligned memory address Date: Mon, 9 Oct 2017 10:28:24 +0200 Message-ID: <3f1e2110-e736-0f53-c8a6-12bfcda433f7@gmail.com> References: <20171009092251.GC5758@HP> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20171009092251.GC5758@HP> Content-Language: en-US Sender: linux-man-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Yubin Ruan , linux-man , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org Cc: mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org List-Id: linux-man@vger.kernel.org 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html