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

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Message-ID: <19b1c8e2-f60e-460d-8249-5db71a708dc9@redhat.com>
Date: Sun, 18 Aug 2024 09:45:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Hugh Dickins <hughd@...gle.com>, Usama Arif <usamaarif642@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, hannes@...xchg.org,
 riel@...riel.com, shakeel.butt@...ux.dev, roman.gushchin@...ux.dev,
 yuzhao@...gle.com, baohua@...nel.org, ryan.roberts@....com, rppt@...nel.org,
 willy@...radead.org, ryncsn@...il.com, ak@...ux.intel.com,
 cerasuolodomenico@...il.com, corbet@....net, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v3 0/6] mm: split underutilized THPs

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