Re: [PATCH 1/1] mm/ksm: trylock the mmap lock in the unstable tree walk

[David Hildenbrand (Arm)]

>> index 49d48d1e0998..3cfb09a926ff 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -820,7 +820,14 @@ static struct page *get_mergeable_page(struct ksm_rmap_item *rmap_item)
>>  	struct folio_walk fw;
>>  	struct folio *folio;
>>  
>> -	mmap_read_lock(mm);
>> +	/*
>> +	 * We trylock because we don't want ksmd to wait for an mm that is
>> +	 * busy changing its memory layout: we prefer to skip this page and
>> +	 * let the next full scan retry it, like the folio trylock in
>> +	 * try_to_merge_one_page().
>> +	 */
>> +	if (!mmap_read_trylock(mm))
>> +		return NULL;
>>  	vma = find_mergeable_vma(mm, addr);
>>  	if (!vma)
>>  		goto out;
>> -- 
>> 2.43.0
> Sorry, NACK

Agreed. Likely the answer is to switch to per-vma locks here. All we do is a
folio_walk_start() to look up a page, that's not too crazy :)

-- 
Cheers,

David