Re: [RFC PATCH 3/3] mm: shm: Use new hugetlb size encoding definitions

[Michal Hocko]

On Thu 27-07-17 14:18:11, Mike Kravetz wrote:
> 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.

yes, add a new one like you did in your first patch

> 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.

agreed

[...]

> >> - 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.

I would use the primary one and only got the arch specific if we ever
need to do arch specific thing.
-- 
Michal Hocko
SUSE Labs