Re: [PATCH v1] mm/memory: move sanity checks in do_wp_page() after mapcount vs. refcount stabilization

[Oscar Salvador]

On Tue, Apr 15, 2025 at 11:50:07AM +0200, David Hildenbrand wrote:
> In __folio_remove_rmap() for RMAP_LEVEL_PMD/RMAP_LEVEL_PUD and with
> CONFIG_PAGE_MAPCOUNT we first decrement the folio mapcount (and
> recompute mapped shared vs. mapped exclusively) to then adjust the
> entire mapcount.
> 
> This means that another process might stumble in do_wp_page() over a
> PTE-mapped PMD folio that is indicated as "exclusively mapped", but still
> has an entire mapcount (PMD mapping), because it is racing with the process
> that is unmapping the folio (PMD mapping). Note that do_wp_page() will
> back off once it detects the remaining folio reference from the process
> that is in the process of unmapping the folio.
> 
> This will trigger the early VM_WARN_ON_ONCE(folio_entire_mapcount(folio))
> check in do_wp_page(), that can easily be reproduced by looping a couple
> of times over allocating a PMD THP, forking a child where we immediately
> unmap it again, and writing in the parent concurrently to the THP.
> 
> [  252.738129][T16470] ------------[ cut here ]------------
> [  252.739267][T16470] WARNING: CPU: 3 PID: 16470 at mm/memory.c:3738 do_wp_page+0x2a75/0x2c00
> [  252.740968][T16470] Modules linked in:
> [  252.741958][T16470] CPU: 3 UID: 0 PID: 16470 Comm: ...
> ...
> [  252.765841][T16470]  <TASK>
> [  252.766419][T16470]  ? srso_alias_return_thunk+0x5/0xfbef5
> [  252.767558][T16470]  ? rcu_is_watching+0x12/0x60
> [  252.768525][T16470]  ? srso_alias_return_thunk+0x5/0xfbef5
> [  252.769645][T16470]  ? srso_alias_return_thunk+0x5/0xfbef5
> [  252.770778][T16470]  ? lock_acquire+0x33/0x80
> [  252.771697][T16470]  ? __handle_mm_fault+0x5e8/0x3e40
> [  252.772735][T16470]  ? __handle_mm_fault+0x5e8/0x3e40
> [  252.773781][T16470]  __handle_mm_fault+0x1869/0x3e40
> [  252.774839][T16470]  handle_mm_fault+0x22a/0x640
> [  252.775808][T16470]  do_user_addr_fault+0x618/0x1000
> [  252.776847][T16470]  exc_page_fault+0x68/0xd0
> [  252.777775][T16470]  asm_exc_page_fault+0x26/0x30
> 
> While we could adjust the sequence in __folio_remove_rmap(), let's rater
> move the mapcount sanity checks after the mapcount vs. refcount
> stabilization phase. With this fix, a simple reproducer is happy.
> 
> While at it, convert the two VM_WARN_ON_ONCE() we are moving to
> VM_WARN_ON_ONCE_FOLIO().
> 
> Reported-by: syzbot+5e8feb543ca8e12e0ede@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lkml.kernel.org/r/67fab4fe.050a0220.2c5fcf.0011.GAE@xxxxxxxxxx
> Fixes: 1da190f4d0a6 ("mm: Copy-on-Write (COW) reuse support for PTE-mapped THP")
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

 

-- 
Oscar Salvador
SUSE Labs