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, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
