Re: [PATCHv2 2/3] i915: convert to new mount API

[Hugh Dickins <hughd@google.com>]

On Mon, 5 Aug 2019, Al Viro wrote:
> On Mon, Aug 05, 2019 at 07:12:55PM +0100, Al Viro wrote:
> > On Tue, Aug 06, 2019 at 01:03:06AM +0900, Sergey Senozhatsky wrote:
> > > tmpfs does not set ->remount_fs() anymore and its users need
> > > to be converted to new mount API.
> > 
> > Could you explain why the devil do you bother with remount at all?
> > Why not pass the right options when mounting the damn thing?
> 
> ... and while we are at it, I really wonder what's going on with
> that gemfs thing - among the other things, this is the only
> user of shmem_file_setup_with_mnt().  Sure, you want your own
> options, but that brings another question - is there any reason
> for having the huge=... per-superblock rather than per-file?

Yes: we want a default for how files of that superblock are to
allocate their pages, without people having to fcntl or advise
each of their files.

Setting aside the weirder options (within_size, advise) and emergency/
testing override (shmem_huge), we want files on an ordinary default
tmpfs (huge=never) to be allocated with small pages (so users with
access to that filesystem will not consume, and will not waste time
and space on consuming, the more valuable huge pages); but files on a
huge=always tmpfs to be allocated with huge pages whenever possible.

Or am I missing your point?  Yes, hugeness can certainly be decided
differently per-file, or even per-extent of file.  That is already
made possible through "judicious" use of madvise MADV_HUGEPAGE and
MADV_NOHUGEPAGE on mmaps of the file, carried over from anon THP.

Though personally I'm averse to managing "f"objects through
"m"interfaces, which can get ridiculous (notably, MADV_HUGEPAGE works
on the virtual address of a mapping, but the huge-or-not alignment of
that mapping must have been decided previously).  In Google we do use
fcntls F_HUGEPAGE and F_NOHUGEPAGE to override on a per-file basis -
one day I'll get to upstreaming those.

Hugh

> 
> After all, the readers of ->huge in mm/shmem.c are
> mm/shmem.c:582:     (shmem_huge == SHMEM_HUGE_FORCE || sbinfo->huge) &&
> 	is_huge_enabled(), sbinfo is an explicit argument
> 
> mm/shmem.c:1799:        switch (sbinfo->huge) {
> 	shmem_getpage_gfp(), sbinfo comes from inode
> 
> mm/shmem.c:2113:                if (SHMEM_SB(sb)->huge == SHMEM_HUGE_NEVER)
> 	shmem_get_unmapped_area(), sb comes from file
> 
> mm/shmem.c:3531:        if (sbinfo->huge)
> mm/shmem.c:3532:                seq_printf(seq, ",huge=%s", shmem_format_huge(sbinfo->huge));
> 	->show_options()
> mm/shmem.c:3880:        switch (sbinfo->huge) {
> 	shmem_huge_enabled(), sbinfo comes from an inode
> 
> And the only caller of is_huge_enabled() is shmem_getattr(), with sbinfo
> picked from inode.
> 
> So is there any reason why the hugepage policy can't be per-file, with
> the current being overridable default?