Re: [PATCH v3 0/6] mm: split underutilized THPs

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Hi Hugh,

2. I don't understand why there needs to be a new PG_partially_mapped
flag, with all its attendant sets and tests and clears all over.  Why
can't deferred_split_scan() detect that case for itself, using the
criteria from __folio_remove_rmap()? I see folio->_nr_pages_mapped
is commented "Do not use outside of rmap and debug code", and
folio_nr_pages_mapped() is currently only used from mm/debug.c; but
using the info already maintained is preferable to adding a PG_flag
(and perhaps more efficient - skips splitting when _nr_pages_mapped
already fell to 0 and folio will soon be freed).

No new users of _nr_pages_mapped if easily/cleanly avoidable, please.

I'm currently cleaning up the final patches that introduce a new kernel config where we will stop maintaining the page->_mapcount for large folios (and consequently have to stop maintaining folio->_nr_pages_mapped).

That's the main reasons for the comment -- at one point in my life I want to be done with that project ;) .

folio->_nr_pages_mapped will still exist and be maintained without the new kernel config enabled. But in the new one, once we'll detect a partial mapping we'll have to flag the folio -- for example as done in this series.

Having two ways of handling that, depending on the kernel config, will not make the code any better.

But I agree that we should look into minimizing the usage of any new such flag: I'd have thought we only have to set the flag once, once we detect a partial mapping ... still have to review that patch more thoroughly.


3. Everything in /sys/kernel/mm/transparent_hugepage/ is about THPs,
so please remove the "thp_" from "thp_low_util_shrinker" -
"shrink_underused" perhaps.  And it needs a brief description in
Documentation/admin-guide/mm/transhuge.rst.

agreed.

--
Cheers,

David / dhildenb





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