Re: [PATCH mmotm 5/5] huge tmpfs: add shmem_pmd_fault()

[Hugh Dickins <hughd@google.com>]

On Sun, 17 Apr 2016, Kirill A. Shutemov wrote:
> On Sat, Apr 16, 2016 at 04:41:33PM -0700, Hugh Dickins wrote:
> > The pmd_fault() method gives the filesystem an opportunity to place
> > a trans huge pmd entry at *pmd, before any pagetable is exposed (and
> > an opportunity to split it on COW fault): now use it for huge tmpfs.
> > 
> > This patch is a little raw: with more time before LSF/MM, I would
> > probably want to dress it up better - the shmem_mapping() calls look
> > a bit ugly; it's odd to want FAULT_FLAG_MAY_HUGE and VM_FAULT_HUGE just
> > for a private conversation between shmem_fault() and shmem_pmd_fault();
> > and there might be a better distribution of work between those two, but
> > prising apart that series of huge tests is not to be done in a hurry.
> > 
> > Good for now, presents the new way, but might be improved later.
> > 
> > This patch still leaves the huge tmpfs map_team_by_pmd() allocating a
> > pagetable while holding page lock, but other filesystems are no longer
> > doing so; and we've not yet settled whether huge tmpfs should (like anon
> > THP) or should not (like DAX) participate in deposit/withdraw protocol.
> > 
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> 
> Just for record: I don't like ->pmd_fault() approach because it results in
> two requests to file system (two shmem_fault() in this case) if we don't
> have a huge page to map: one for huge page (failed) and then one for small.
> I think this case should be rather common: all mounts without huge pages
> enabled. I expect performance regression from this too.

Yes, I did consider that when making the switchover.  But it's only
when pmd_none(*pmd), not the other 511 times; and the caches have been
primed for the pte fallback.  So I didn't expect it to matter, and to be
outweighed by having map_pages() back in its old position.  Ah, you'll
point out that map_pages() makes it a smaller ratio than 511:1.

But if someone speeds up pmd_fault(), or replaces it by a better strategy,
so much the better - I found it a little odd, doing two very different
things, one of which (splitting) must be done in a non-fault context too.

Anyway, I await judgement from the robot.

And note your point about regressing mounts without huge pages enabled:
maybe I should add an early VM_FAULT_FALLBACK for that case, or perhaps
it will end up in the vma flags instead of my shmem_mapping() check.

Hugh

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