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

[Danilo Krummrich <dakr@redhat.com>]

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.

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