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SubjectRe: [patch for-5.3 0/4] revert immediate fallback to remote hugepages
On Wed, 2 Oct 2019, Michal Hocko wrote:

> > > If
> > > hugetlb wants to stress this to the fullest extent possible, it already
> > > appropriately uses __GFP_RETRY_MAYFAIL.
> >
> > Which doesn't work anymore right now, and should again after this patch.
> I didn't get to fully digest the patch Vlastimil is proposing. (Ab)using
> __GFP_NORETRY is quite subtle but it is already in place with some
> explanation and a reference to THPs. So while I am not really happy it
> is at least something you can reason about.

It's a no-op:

/* Do not loop if specifically requested */
if (gfp_mask & __GFP_NORETRY)
goto nopage;

* Do not retry costly high order allocations unless they are
if (costly_order && !(gfp_mask & __GFP_RETRY_MAYFAIL))
goto nopage;

So I'm not sure we should spend too much time discussing a hunk of a patch
that doesn't do anything.

> b39d0ee2632d ("mm, page_alloc: avoid expensive reclaim when compaction
> may not succeed") on the other hand has added a much more wider change
> which has clearly broken hugetlb and any __GFP_RETRY_MAYFAIL user of
> pageblock_order sized allocations. And that is much worse and something
> I was pointing at during the review and those concerns were never really
> addressed before merging.
> In any case this is something to be fixed ASAP. Do you have any better
> proposa? I do not assume you would be proposing yet another revert.

I thought Mike Kravetz said[*] that hugetlb was not negatively affected by
this? We could certainly disregard this logic for __GFP_RETRY_MAYFAIL if
anybody is relying on excessive reclaim ("swap storms") that does not
allow compaction to make forward progress for some reason.


If not, for the purposes of this conversation we can disregard
__GFP_NORETRY per the above because thp does not use __GFP_RETRY_MAYFAIL.

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