Re: [RFC PATCH] mm: shm: round up tmpfs size to huge page size when huge=always

[Yang Shi]

On 10/8/17 5:56 AM, Kirill A. Shutemov wrote:
> On Sat, Oct 07, 2017 at 04:22:10AM +0800, Yang Shi wrote:
> > When passing "huge=always" option for mounting tmpfs, THP is supposed to
> > be allocated all the time when it can fit, but when the available space is
> > smaller than the size of THP (2MB on x86), shmem fault handler still tries
> > to allocate huge page every time, then fallback to regular 4K page
> > allocation, i.e.:
> >
> > 	# mount -t tmpfs -o huge,size=3000k tmpfs /tmp
> > 	# dd if=/dev/zero of=/tmp/test bs=1k count=2048
> > 	# dd if=/dev/zero of=/tmp/test1 bs=1k count=2048
> >
> > The last dd command will handle 952 times page fault handler, then exit
> > with -ENOSPC.
> >
> > Rounding up tmpfs size to THP size in order to use THP with "always"
> > more efficiently. And, it will not wast too much memory (just allocate
> > 511 extra pages in worst case).
>
> Hm. I don't think it's good idea to silently increase size of fs.

How about printing a warning to say the filesystem is resized?

> Maybe better just refuse to mount with huge=always for too small fs?

It sounds fine too. When mounting tmpfs with "huge=always", if the size 
is not THP size aligned, it just can refuse to mount, then show warning 
about alignment restriction.

Thanks,
Yang

>