Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 26 Jun 2023 16:27:54 +0200 | Subject | Re: [PATCH v2 14/16] maple_tree: Refine mas_preallocate() node calculations | From | Danilo Krummrich <> |
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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?
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