Re: [PATCH v2 14/16] maple_tree: Refine mas_preallocate() node calculations

[Danilo Krummrich <dakr@redhat.com>]

On 6/26/23 16:49, Peng Zhang wrote:
> 
> 
> 在 2023/6/26 22:27, Danilo Krummrich 写道:
>> On 6/26/23 15:19, Matthew Wilcox wrote:
>>> On Mon, Jun 26, 2023 at 02:38:06AM +0200, Danilo Krummrich wrote:
>>>> On the other hand, unless I miss something (and if so, please let me 
>>>> know),
>>>> something is bogus with the API then.
>>>>
>>>> While the documentation of the Advanced API of the maple tree 
>>>> explicitly
>>>> claims that the user of the API is responsible for locking, this 
>>>> should be
>>>> limited to the bounds set by the maple tree implementation. Which 
>>>> means, the
>>>> user must decide for either the internal (spin-) lock or an external 
>>>> lock
>>>> (which possibly goes away in the future) and acquire and release it
>>>> according to the rules maple tree enforces through lockdep checks.
>>>>
>>>> Let's say one picks the internal lock. How is one supposed to ensure 
>>>> the
>>>> tree isn't modified using the internal lock with mas_preallocate()?
>>>>
>>>> Besides that, I think the documentation should definitely mention this
>>>> limitation and give some guidance for the locking.
>>>>
>>>> Currently, from an API perspective, I can't see how anyone not 
>>>> familiar with
>>>> the implementation details would be able to recognize this limitation.
>>>>
>>>> In terms of the GPUVA manager, unfortunately, it seems like I need 
>>>> to drop
>>>> the maple tree and go back to using a rb-tree, since it seems there 
>>>> is no
>>>> sane way doing a worst-case pre-allocation that does not suffer from 
>>>> this
>>>> limitation.
>>>
>>> I haven't been paying much attention here (too many other things going
>>> on), but something's wrong.
>>>
>>> First, you shouldn't need to preallocate.  Preallocation is only there
>>
>> Unfortunately, I think we really have a case where we have to. 
>> Typically GPU mappings are created in a dma-fence signalling critical 
>> path and that is where such mappings need to be added to the maple 
>> tree. Hence, we can't do any sleeping allocations there.
>>
>>> for really gnarly cases.  The way this is *supposed* to work is that
>>> the store walks down to the leaf, attempts to insert into that leaf
>>> and tries to allocate new nodes with __GFP_NOWAIT.  If that fails,
>>> it drops the spinlock, allocates with the gfp flags you've specified,
>>> then rewalks the tree to retry the store, this time with allocated
>>> nodes in its back pocket so that the store will succeed.
>>
>> You are talking about mas_store_gfp() here, right? And I guess, if the 
>> tree has changed while the spinlock was dropped and even more nodes 
>> are needed it just retries until it succeeds?
>>
>> But what about mas_preallocate()? What happens if the tree changed in 
>> between mas_preallocate() and mas_store_prealloc()? Does the latter 
>> one fall back to __GFP_NOWAIT in such a case? I guess not, since 
>> mas_store_prealloc() has a void return type, and __GFP_NOWAIT could 
>> fail as well.
> mas_store_prealloc() will fallback to __GFP_NOWAIT and issue a warning.
> If __GFP_NOWAIT allocation fails, BUG_ON() in mas_store_prealloc() will
> be triggered.

Ok, so this is an absolute last resort and surely should not be relied on.

I think the maple tree should either strictly enforce (through locking 
policy) that this can never happen or if API wise it is OK not to lock 
these two is legit, return an error code rather then issue a warning and 
even worse call BUG_ON() in case it can't fix things up.

- Danilo

> 
>>
>> So, how to use the internal spinlock for mas_preallocate() and 
>> mas_store_prealloc() to ensure the tree can't change?
>>
>