Re: [PATCH 2/2] mm: THP page cache support for ppc64

[Hugh Dickins]

On Fri, 11 Nov 2016, Kirill A. Shutemov wrote:
> On Fri, Nov 11, 2016 at 05:42:11PM +0530, Aneesh Kumar K.V wrote:
> > 
> > doing this in do_set_pmd keeps this closer to where we set the pmd. Any
> > reason you thing we should move it higher up the stack. We already do
> > pte_alloc() at the same level for a non transhuge case in
> > alloc_set_pte().
> 
> I vaguely remember Hugh mentioned deadlock of allocation under page-lock vs.
> OOM-killer (or something else?).

You remember well.  It was indeed the OOM killer, but in particular due
to the way it used to wait for a current victim to exit, and that exit
could be delayed forever by the way munlock_vma_pages_all() goes to lock
each page in a VM_LOCKED area - a pity if one of them is the page we
hold locked while servicing a fault and need to allocate a pagetable.

> 
> If the deadlock is still there it would be matter of making preallocation
> unconditional to fix the issue.

I think enough has changed at the OOM killer end that the deadlock is
no longer there.  I haven't kept up with all the changes made recently,
but I think we no longer wait for a unique victim to exit before trying
another (reaped mms set MMF_OOM_SKIP); and the OOM reaper skips over
VM_LOCKED areas to avoid just such a deadlock.

It's still silly that munlock_vma_pages_all() should require page lock
on each of those pages; but neither Michal nor I have had time to
revisit our attempts to relieve that requirement - mlock.c is not easy.

> 
> But what you propose about doesn't make situation any worse. I'm fine with
> that.

Yes, I think that's right: if there is a problem, then it would already
be problem since alloc_set_pte() was created; but we've seen no reports.

Hugh