Re: [PATCH 06/16] huge tmpfs: shmem_is_huge(vma, inode, index)

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.

quoted

quoted

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