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Date:   Thu, 5 Aug 2021 22:43:37 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Yang Shi <shy828301@gmail.com>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
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        Mike Kravetz <mike.kravetz@oracle.com>,
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Subject: Re: [PATCH 06/16] huge tmpfs: shmem_is_huge(vma, inode, index)
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On Thu, 5 Aug 2021, Yang Shi wrote:
> 
> By rereading the code, I think you are correct. Both cases do work
> correctly without leaking. And the !CONFIG_NUMA case may carry the
> huge page indefinitely.
> 
> I think it is because khugepaged may collapse memory for another NUMA
> node in the next loop, so it doesn't make too much sense to carry the
> huge page, but it may be an optimization for !CONFIG_NUMA case.

Yes, that is its intention.

> 
> However, as I mentioned in earlier email the new pcp implementation
> could cache THP now, so we might not need keep this convoluted logic
> anymore. Just free the page if collapse is failed then re-allocate
> THP. The carried THP might improve the success rate a little bit but I
> doubt how noticeable it would be, may be not worth for the extra
> complexity at all.

It would be great if the new pcp implementation is good enough to
get rid of khugepaged's confusing NUMA=y/NUMA=n differences; and all
the *hpage stuff too, I hope.  That would be a welcome cleanup.

> > > Collapse failure is not uncommon and leaking huge pages gets noticed.

After writing that, I realized how I'm almost always testing a NUMA=y
kernel (though on non-NUMA machines), and seldom try the NUMA=n build.
So did so to check no leak, indeed; but was surprised, when comparing
vmstats, that the NUMA=n run had done 5 times as much thp_collapse_alloc
as the NUMA=y run.  I've merely made a note to look into that one day:
maybe it was just a one-off oddity, or maybe the incrementing of stats
is wrong down one path or the other.

Hugh