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

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Message-ID: <Zfnd_w0ZLOVhgACt@casper.infradead.org>
Date: Tue, 19 Mar 2024 18:48:31 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Oscar Salvador <osalvador@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Michal Hocko <mhocko@...e.com>,
	Vlastimil Babka <vbabka@...e.cz>, 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 Tue, Mar 19, 2024 at 07:32:12PM +0100, Oscar Salvador wrote:
> Upon migration, new allocated pages are being given the handle of the old
> pages. This is problematic because it means that for the stack which
> allocated the old page, we will be substracting the old page + the new one
> when that page is freed, creating an accounting imbalance.
> 
> Fix this by adding a new migrate_handle in the page_owner struct, and
> record the handle that allocated the new page in __folio_copy_owner().
> Upon freeing, we check whether we have a migrate_handle, and if we do,
> we use migrate_handle for dec_stack_record_count(), which will
> subtract those pages from its right handle.

Is this the right way to fix this problem?  I would have thought we'd
be better off accounting this as migration freeing the old page and
allocating the new page.  If I understand correctly, this is the code
which says "This page was last allocated by X and freed by Y", and I
would think that being last freed (or allocated) by the migration code
would be a very nice hint about where a problem might stem from.