Revision 8a8683ad9ba48b4b52a57f013513d1635c1ca5c4 authored by Huang Ying on 06 March 2020, 06:28:29 UTC, committed by Linus Torvalds on 06 March 2020, 13:06:09 UTC
In set_pmd_migration_entry(), pmdp_invalidate() is used to change PMD
atomically. But the PMD is read before that with an ordinary memory
reading. If the THP (transparent huge page) is written between the PMD
reading and pmdp_invalidate(), the PMD dirty bit may be lost, and cause
data corruption. The race window is quite small, but still possible in
theory, so need to be fixed.
The race is fixed via using the return value of pmdp_invalidate() to get
the original content of PMD, which is a read/modify/write atomic
operation. So no THP writing can occur in between.
The race has been introduced when the THP migration support is added in
the commit 616b8371539a ("mm: thp: enable thp migration in generic path").
But this fix depends on the commit d52605d7cb30 ("mm: do not lose dirty
and accessed bits in pmdp_invalidate()"). So it's easy to be backported
after v4.16. But the race window is really small, so it may be fine not
to backport the fix at all.
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: <stable@vger.kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Link: http://lkml.kernel.org/r/20200220075220.2327056-1-ying.huang@intel.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 8b272b3
percpu-rw-semaphore.txt
====================
Percpu rw semaphores
====================
Percpu rw semaphores is a new read-write semaphore design that is
optimized for locking for reading.
The problem with traditional read-write semaphores is that when multiple
cores take the lock for reading, the cache line containing the semaphore
is bouncing between L1 caches of the cores, causing performance
degradation.
Locking for reading is very fast, it uses RCU and it avoids any atomic
instruction in the lock and unlock path. On the other hand, locking for
writing is very expensive, it calls synchronize_rcu() that can take
hundreds of milliseconds.
The lock is declared with "struct percpu_rw_semaphore" type.
The lock is initialized percpu_init_rwsem, it returns 0 on success and
-ENOMEM on allocation failure.
The lock must be freed with percpu_free_rwsem to avoid memory leak.
The lock is locked for read with percpu_down_read, percpu_up_read and
for write with percpu_down_write, percpu_up_write.
The idea of using RCU for optimized rw-lock was introduced by
Eric Dumazet <eric.dumazet@gmail.com>.
The code was written by Mikulas Patocka <mpatocka@redhat.com>
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