linux-kernel - Re: [PATCH v2 2/2] mm,page_owner: Fix accounting of pages when migrating

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Date: Thu, 21 Mar 2024 12:07:02 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Matthew Wilcox <willy@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michal Hocko <mhocko@...e.com>, Marco Elver <elver@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [PATCH v2 2/2] mm,page_owner: Fix accounting of pages when
 migrating

On Thu, Mar 21, 2024 at 11:50:36AM +0100, Vlastimil Babka wrote:
> Yeah I think we could keep that logic.

I am all for keeping it.

> But we could also simply subtract the refcount of the old handle (the
> "allocated for migration") in __folio_copy_owner() no? Then we wouldn't need
> the extra migrate_handle.

Since new_page will have the old handle pointing to the old stack after
the call, we
could uncharge the old_page to the migrate_stack, which new_page->_handle holds
before it gets changed.

So we basically swap it.

It could work, but I kinda have a bittersweet feeling here.
I am trying to work towards to reduce the number of lookups in the
hlist, but for the approach described above I would need to lookup
the stack for new_page->handle in order to substract the page.

OTHO, I understand that adding migrate_handle kinda wasted memory.
16MB for 16GB of memory.

> Also we might have more issues here. Most page owner code takes care to set
> everything for all pages within a folio, but __folio_copy_owner() and
> __set_page_owner_migrate_reason() don't.

I did not check deeply but do not we split the folio upon migration
in case it is large?
Which means we should reach split_page_owner() before the copy takes
place. Do I get it right?

-- 
Oscar Salvador
SUSE Labs