Received-SPF: pass (google.com: domain of linux-kernel+bounces-109919-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Thu, 21 Mar 2024 12:07:02 +0100
From: Oscar Salvador <osalvador@suse.de>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Michal Hocko <mhocko@suse.com>, Marco Elver <elver@google.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Alexander Potapenko <glider@google.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Subject: Re: [PATCH v2 2/2] mm,page_owner: Fix accounting of pages when
 migrating
Message-ID: <ZfwU1mGQKejNaKIk@localhost.localdomain>
References: <20240319183212.17156-1-osalvador@suse.de>
 <20240319183212.17156-3-osalvador@suse.de>
 <Zfnd_w0ZLOVhgACt@casper.infradead.org>
 <ZfptWDsfdxBltN6T@localhost.localdomain>
 <ed41d5cf-d068-412d-b7bb-5468df2fefb7@suse.cz>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ed41d5cf-d068-412d-b7bb-5468df2fefb7@suse.cz>

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