linux-kernel - Re: [linus:master] [mm] d99e3140a4: BUG:KCSAN:data-race_in_folio_remove_rmap_ptes/print

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Date: Tue, 28 May 2024 09:47:47 +0200
From: Oscar Salvador <osalvador@...e.de>
To: David Hildenbrand <david@...hat.com>
Cc: kernel test robot <oliver.sang@...el.com>,
	Matthew Wilcox <willy@...radead.org>, oe-lkp@...ts.linux.dev,
	lkp@...el.com, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [linus:master] [mm] d99e3140a4:
 BUG:KCSAN:data-race_in_folio_remove_rmap_ptes/print_report

On Tue, May 28, 2024 at 09:43:39AM +0200, David Hildenbrand wrote:
> Likely that's just a page_type check racing against concurrent
> mapcount changes.
> 
> In __folio_rmap_sanity_checks() we check
> 	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);

Yeah, and that "collides" with

last = atomic_add_negative(-1, &page->_mapcount)

from __folio_remove_rmap.

> To make sure we don't get hugetlb folios in the wrong rmap code path. That
> can easily race with concurrent mapcount changes, just like any other
> page_type checks that end up in folio_test_type/page_has_type e.g., from
> PFN walkers.
> 
> Load tearing in these functions shouldn't really result in false positives
> (what we care about), but READ_ONCE shouldn't hurt or make a difference.
> 
> 
> From b03dc9bf27571442d886d8da624a4e4f737433f2 Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@...hat.com>
> Date: Tue, 28 May 2024 09:37:20 +0200
> Subject: [PATCH] mm: read page_type using READ_ONCE
> 
> KCSAN complains about possible data races: while we check for a
> page_type -- for example for sanity checks -- we might concurrently
> modify the mapcount that overlays page_type.
> 
> Let's use READ_ONCE to avoid laod tearing (shouldn't make a difference)
> and to make KCSAN happy.
> 
> Note: nothing should really be broken besides wrong KCSAN complaints.
> 
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202405281431.c46a3be9-lkp@intel.com
> Signed-off-by: David Hildenbrand <david@...hat.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

Thanks!

-- 
Oscar Salvador
SUSE Labs