linux-kernel - Re: [PATCH v2 01/10] maple_tree: Rework mtree_alloc

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Message-ID: <d3e03ee2-284e-1e32-6273-cf21b4afcf99@bytedance.com>
Date:   Thu, 18 May 2023 14:10:09 +0800
From:   Peng Zhang <zhangpeng.00@...edance.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     Peng Zhang <zhangpeng.00@...edance.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        maple-tree@...ts.infradead.org
Subject: Re: [PATCH v2 01/10] maple_tree: Rework mtree_alloc_{range,rrange}()



在 2023/5/18 02:17, Liam R. Howlett 写道:
> * Peng Zhang <zhangpeng.00@...edance.com> [230517 04:58]:
>> Use mas_empty_area{_rev}() to refactor mtree_alloc_{range,rrange}()
>>
>> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
>> ---
>>   lib/maple_tree.c | 63 ++++++++++++++++++++++++------------------------
>>   1 file changed, 32 insertions(+), 31 deletions(-)
>>
>> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
>> index 4eb220008f72..e1820e90f167 100644
>> --- a/lib/maple_tree.c
>> +++ b/lib/maple_tree.c
>> @@ -6493,32 +6493,31 @@ int mtree_alloc_range(struct maple_tree *mt, unsigned long *startp,
>>   {
>>   	int ret = 0;
>>   
>> -	MA_STATE(mas, mt, min, min);
>> +	MA_STATE(mas, mt, 0, 0);
>>   	if (!mt_is_alloc(mt))
>>   		return -EINVAL;
>>   
>>   	if (WARN_ON_ONCE(mt_is_reserved(entry)))
>>   		return -EINVAL;
>>   
>> -	if (min > max)
>> -		return -EINVAL;
>> -
>> -	if (max < size)
>> -		return -EINVAL;
>> -
>> -	if (!size)
>> -		return -EINVAL;
>> -
>>   	mtree_lock(mt);
>>   retry:
>> -	mas.offset = 0;
>> -	mas.index = min;
>> -	mas.last = max - size + 1;
>> -	ret = mas_alloc(&mas, entry, size, startp);
>> -	if (mas_nomem(&mas, gfp))
>> -		goto retry;
>> -
>> +	ret = mas_empty_area(&mas, min, max, size);
>> +	if (!ret) {
>> +		mas_insert(&mas, entry);
>> +		/*
>> +		 * mas_nomem() may release the lock, causing the allocated area
>> +		 * to be unavailable, so try to allocate a free area again.
>> +		 */
>> +		if (mas_nomem(&mas, gfp))
>> +			goto retry;
>> +	}
>>   	mtree_unlock(mt);
>> +	if (!ret) {
> 
> Checking for a mas_is_err() should probably be outside if (!ret)
> statement.  If mas_insert() returns something besides ENOMEM, we will
> not detect the error.  I'm not sure if this is possible today since this
> should never return an -EEXISTS, but having it this way doesn't add much
> to the overhead.
I don't think there will be error that can't be detected here.
In fact, there are two sources of errors:

1. mas_empty_area(), the error number is in the variable ret,
    and may also be in mas->node, but ret must contain all errors.

2. mas_insert(), the error number is in mas->node

When we check errors, we should first check errors from
mas_empty_area(). If there is no error in mas_empty_area(), we
will check errors from mas_insert().
So, mas_is_err() is inside the if (!ret) statement, no problem here.

Of course, even if mas_insert() returns -EEXISTS, it can be detected
under the current encoding, because "if (!ret)" is true in this case.
But I don't think this can happen, if it happens, it's a bug of maple
tree.

I don't think it's good to put mas_is_err() outside, because the error
number stored in mas->node may come from mas_empty_area(). We should use
the ret variable to detect the error from mas_empty_area() first.
> 
>> +		if (mas_is_err(&mas))
>> +			return xa_err(mas.node);
>> +		*startp = mas.index;
>> +	}
>>   	return ret;
>>   }
>>   EXPORT_SYMBOL(mtree_alloc_range);
>> @@ -6529,29 +6528,31 @@ int mtree_alloc_rrange(struct maple_tree *mt, unsigned long *startp,
>>   {
>>   	int ret = 0;
>>   
>> -	MA_STATE(mas, mt, min, max - size + 1);
>> +	MA_STATE(mas, mt, 0, 0);
>>   	if (!mt_is_alloc(mt))
>>   		return -EINVAL;
>>   
>>   	if (WARN_ON_ONCE(mt_is_reserved(entry)))
>>   		return -EINVAL;
>>   
>> -	if (min > max)
>> -		return -EINVAL;
>> -
>> -	if (max < size - 1)
>> -		return -EINVAL;
>> -
>> -	if (!size)
>> -		return -EINVAL;
>> -
>>   	mtree_lock(mt);
>>   retry:
>> -	ret = mas_rev_alloc(&mas, min, max, entry, size, startp);
>> -	if (mas_nomem(&mas, gfp))
>> -		goto retry;
>> -
>> +	ret = mas_empty_area_rev(&mas, min, max, size);
>> +	if (!ret) {
>> +		mas_insert(&mas, entry);
>> +		/*
>> +		 * mas_nomem() may release the lock, causing the allocated area
>> +		 * to be unavailable, so try to allocate a free area again.
>> +		 */
>> +		if (mas_nomem(&mas, gfp))
>> +			goto retry;
>> +	}
>>   	mtree_unlock(mt);
>> +	if (!ret) {
> 
> Same here.
> 
>> +		if (mas_is_err(&mas))
>> +			return xa_err(mas.node);
>> +		*startp = mas.index;
>> +	}
>>   	return ret;
>>   }
>>   EXPORT_SYMBOL(mtree_alloc_rrange);
>> -- 
>> 2.20.1
>>