Re: [PATCH RFC 00/12] mm/vmalloc: migrate vmap_area indexing from rb-tree to maple-tree

[Pranjal Arya <pranjal.arya@oss.qualcomm.com>]

On 6/18/2026 3:36 PM, Uladzislau Rezki wrote:
> On Tue, Jun 16, 2026 at 07:07:57PM +0100, Matthew Wilcox wrote:
>> On Mon, Jun 15, 2026 at 11:52:22AM +0200, Uladzislau Rezki wrote:
>>> On Sun, Jun 14, 2026 at 12:15:28AM +0100, Matthew Wilcox wrote:
>>>> What I don't understand is why you maintain a separate "free" tree.
>>>> It should not be necessary any more, but maybe you tried removing it
>>>> already and found a performance problem?
>>>
>>> We maintain it in order to split several entities. That prevents
>>> interfering between allocated data and vmap-free-space manager.
>>> So in that case one context can easily access allocated data, for
>>> example vread iterator, etc., whereas another can do an allocation.
>>>
>>> So by splitting parts i minimize lock-contention.
>>
>> Sure, but there are many ways to reduce lock contention.  One is to not
>> take locks at all; the maple tree is RCU-safe, so you can read the tree
>> holding only the RCU read lock, as long as you obey the RCU rules.
>>
>> Specifically:
>>  - Write side has to RCU-free the objects that are stored in the tree
>>  - Read side has to trylock the objects it finds (and retry the walk
>>    if the trylock fails)
>>  - Read side can see a mixture of objects if the tree is changed while
>>    it is reading, but for any given index in the tree it is guaranteed
>>    to see one of the objects which has been referred to by that index.
>>    That is, if the write side overwrites an index that referred to
>>    object A with object B, the reader will see either object A or B.
>>    It will not see NULL and it will not see any other object.
>>  - If the write side stores both object C and object D in the tree,
>>    the read side may see neither, both, only C or only D.
>>
> Some thoughts about it.
> 
> Having the tree which is RCU safe is good for sure. We can benefit from
> at least in the: vmallocinfo scanning/dumping, possibly in the vread_iter()
> when access to /proc/kcore and other places(which i need to check carefully).
> But this is for read-only traversal.
> 

I agree. The RCU safe busy tree is a foundation that enables lockless
read only traversal. Will implement it in next patchset.

> Switching to gap-based approach requires quite a bit of refactoring and it
> should be a full switch without any hybrid schemes or mixes. I expect that
> we remove more code then adding because of some parts will become hidden
> like lookups/reserving range/erase, etc which is good.
> 
> - replacing free_vmap_area to maple-tree gap based approach;
> - rewriting pcpu-allocator which lives in the end of vmalloc space;
> - refactoring per-cpu allocator which is also part of vmalloc space;
> - vread iterator;
> - vmalloc dump path;
> - vmap_node logic(use gap-reserve to minimize contention);
> - and more...
> 
> To me such rewrite makes sense if we end up in something structural not
> just because maple tree exists. The criteria i would go with are: at least
> same performance level, remove more then add, the design stays at least in
> same good shape.
> 
> There are some drawback i am thinking of. One of them is maple insert path,
> mas_store_gfp()? First we need to find an empty area, then set-range and do
> mas_store_gfp() that uses gfp flag for its internal allocation. If we are
> under spin-lock sleeping is not possible, using NOWAIT or ATOMIC is not a
> case thus we should somehow pre-allocate outside the lock and store the range
> without any allocation.
> 

I am planning to have following approach on this:

  1. mas_preallocate(GFP_NOWAIT | __GFP_NOWARN) + mas_store_prealloc
     will be fast path. The preallocate attempt will be non sleeping and,
     if it succeeds, the subsequent store won't require allocation.

  2. mas_store_gfp(GFP_ATOMIC | __GFP_NOWARN) fallback: if
     preallocate fails (rare but possible under memory pressure),
     GFP_ATOMIC will make a non sleeping allocation attempt inline.

  3. vmap_retry_list recovery queue: if both above fail, the VA will be
     added to a non indexed retry list.  The allocator will scan the list
     on subsequent calls, and purge workers will drain it. This will avoid
     any leak or panic under sustained slab pressure.

Neither GFP_NOWAIT nor GFP_ATOMIC can sleep under a spinlock. The retry
will queue provide the correctness backstop to avoid GFP_KERNEL
blocking inside the lock.

> The allocator operation:
>  - finds an empty range;
>  - publishes VA that blocks that range.
> 
> those two have to be serialized among other writes. Otherwise two CPUs can use
> same empty range and both try to reserve them. If preallocate outside the lock,
> the "alloc" side has to validate that a selected range is still empty and only
> then store VA to block the range.
> 

Sure. Both will be called under a lock. Holding both under the same lock
is the better serialization approach. No two concurrent allocators will
observe the same gap and both succeed. In the next patchset, I'll
explicitly add a comment on the lock declaration describing this approach
in commit message.

> I think it is worth to prototype something to see how it would go. I may be
> missing something for sure.
> 
> Thank you for your input!
> 
> --
> Uladzislau Rezki

Thank you for the detailed design questions. This will make upcoming
patchset substantially cleaner than the RFC.

BR,
Pranjal