On 07/27/2017 12:50 AM, Michal Hocko wrote: > On Wed 26-07-17 10:39:30, Mike Kravetz wrote: >> On 07/26/2017 03:07 AM, Michal Hocko wrote: >>> On Wed 26-07-17 11:53:38, Michal Hocko wrote: >>>> On Mon 17-07-17 15:28:01, Mike Kravetz wrote: >>>>> Use the common definitions from hugetlb_encode.h header file for >>>>> encoding hugetlb size definitions in shmget system call flags. In >>>>> addition, move these definitions to the from the internal to user >>>>> (uapi) header file. >>>> >>>> s@to the from@from@ >>>> >>>>> >>>>> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx> >>>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> >>>> >>>> with s@HUGETLB_FLAG_ENCODE__16GB@HUGETLB_FLAG_ENCODE_16GB@ >>>> >>>> Acked-by: Michal Hocko <mhocko@xxxxxxxx> >>> >>> Btw. man page mentions only 2MB and 1GB, we should document others and >>> note that each arch might support only subset of them >> >> Thanks for looking at these Michal. >> BTW, those definitions below are wrong. They should be SHM_HUGE_*. :( > > Ups, and I completely missed that. > >> In the overview of this RFC, I mentioned still needing to address the >> comment from Aneesh about splitting SHM_HUGE_* definitions into arch >> specific header files. This is how it is done for mmap. If an arch >> supports multiple huge page sizes, the 'asm/mman.h' contains definitions >> for those sizes. There will be a bit of churn (such as header file >> renaming) to do this for shm as well. So, I keep going back and forth >> asking myself 'is it worth it'? > > Why cannot we use a generic header? Btw. I think it would be better for > MMAP definitions as well. I assume you are asking about a uapi asm-generic header file? Currently mmap has two such files: mman.h and mman-common.h. In order to get the definitions in such files, arch specific header files must #include the asm-generic headers. There are arch specific mmap headers today that do not include either of the asm-generic headers. And, they have their own definitions for MAP_HUGE_SHIFT. So, it seems we can not use one of the existing mmap asm-generic header files. Rather, we would need to create a new one and have that included by all arch specific files. However, ALL the MAP_HUGE_* definitions in all the arch specific and asm-generic header files are the same. It would be possible to just put all those MAP_HUGE_* definitions in the primary uapi header file (include/uapi/linux/mman.h). If there was ever a need for arch specific values in the future, we could split them out at that time. >> Some things to consider. >> >> - We should be consistent between mmap and shm. Also remember, that I >> will propose adding the same type of encoding to memfd_create. So, >> three system calls will use the encoding. They should be consistent. > > agreed > >> - Adding the arch specific definitions seems the 'most correct', as a >> user can not use a definition not supported by the arch. However, >> even if an arch supports a huge page size it does not mean that the >> running kernel supports that size. Therefore, the folllowing is in >> the man page. >> "The range of huge page sizes that are supported by the system >> can be discovered by listing the subdirectories in >> /sys/kernel/mm/hugepages." > > Doesn't the respective call return -EINVAL on the unsupported hugepage > size? Yes, it does. >> - Another alternative is to make all known huge page sizes available >> to all users. This is 'easier' as the definitions can likely reside >> in a common header file. The user will need to determine what >> huge page sizes are supported by the running kernel as mentioned in >> the man page. > > yes I think this makes more sense. Ok, thanks. The only remaining question is what kind of common header to use: 1) An asm-generic header file in case there may be arch specific differences in the future. 2) Use the primary uapi header file in include/uapi/linux/mman|shm.h. -- Mike Kravetz -- 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>
