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SubjectRe: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()
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Hi Kirill,

On 2023/3/9 06:39, Kirill Tkhai wrote:
> On 07.03.2023 09:56, Qi Zheng wrote:
>> Now there are no readers of shrinker_rwsem, so
>> synchronize_shrinkers() does not need to hold the
>> writer of shrinker_rwsem to wait for all running
>> shinkers to complete, synchronize_srcu() is enough.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>> mm/vmscan.c | 8 ++------
>> 1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7aaf6f94ac1b..ac7ab4aa344f 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -796,15 +796,11 @@ EXPORT_SYMBOL(unregister_shrinker);
>> /**
>> * synchronize_shrinkers - Wait for all running shrinkers to complete.
>> *
>> - * This is equivalent to calling unregister_shrink() and register_shrinker(),
>> - * but atomically and with less overhead. This is useful to guarantee that all
>> - * shrinker invocations have seen an update, before freeing memory, similar to
>> - * rcu.
>> + * This is useful to guarantee that all shrinker invocations have seen an
>> + * update, before freeing memory.
>> */
>> void synchronize_shrinkers(void)
>> {
>> - down_write(&shrinker_rwsem);
>> - up_write(&shrinker_rwsem);
>> atomic_inc(&shrinker_srcu_generation);
>> synchronize_srcu(&shrinker_srcu);
>> }
>
> Just curious, callers of synchronize_shrinkers() don't want to have parallel register_shrinker() and unregister_shrink() are completed?
> Here we only should wait for parallel shrink_slab(), correct?

I think yes.

The synchronize_shrinkers() is currently only used by TTM pool.

In TTM pool, a shrinker named "drm-ttm_pool" is registered, and
the scan_objects callback will pick an entry from its own shrinker_list:

ttm_pool_shrink
--> spin_lock(&shrinker_lock);
pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
list_move_tail(&pt->shrinker_list, &shrinker_list);
spin_unlock(&shrinker_lock);

These entries have been removed from shrinker_list before calling
synchronize_shrinkers():

ttm_pool_fini
--> ttm_pool_type_fini
--> spin_lock(&shrinker_lock);
list_del(&pt->shrinker_list);
spin_unlock(&shrinker_lock);
synchronize_shrinkers

So IIUC, we only need to wait for the parallel shrink_slab() here. Like
its comment says:

/* We removed the pool types from the LRU, but we need to also make sure
* that no shrinker is concurrently freeing pages from the pool.
*/

+ CC: Christian König :)

Thanks,
Qi

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