Re: [bug report] memory leak of xa_node in collapse_file() when rollbacks

[Shardul Bankar <shardul.b@mpiricsoftware.com>]

On Thu, 2025-12-18 at 21:11 +0800, Jinjiang Tu wrote:
> 
> 在 2025/12/18 20:49, David Hildenbrand (Red Hat) 写道:
>  
> > > 
> > 
> >  Thanks for checking. I thought that was also discussed as part of
> > the other fix. 
> >  
> >  See [2] where we have 
> >  
> >  "Note: This fixes the leak of pre-allocated nodes. A separate fix
> > will 
> >  be needed to clean up empty nodes that were inserted into the tree
> > by 
> >  xas_create_range() but never populated." 
> >  
> >  Is that the issue you are describing? (sounds like it, but I only
> > skimmed over the details). 
> >  
> >  CCing Shardul. 
> Yes, the same issue. As I descirbed in the first email:
> "
> At first, I tried to destory the empty nodes when collapse_file()
> goes to rollback path. However,
> collapse_file() only holds xarray lock and may release the lock, so
> we couldn't prevent concurrent
> call of collapse_file(), so the deleted empty nodes may be needed by
> other collapse_file() calls. 
> "

Hi David, Jinjiang,

As Jinjiang mentioned, this appears to address what I had originally
referred to in the "Note:" in [1].

Just to clarify the context of the "Note:", that was based on my
assumption at the time that such empty nodes would be considered leaks.
After Dev’s feedback in [2]:
"No "fix" is needed in this case, the empty nodes are there in the tree
and there is no leak."

and looking at the older discussion in [3]:
"There's nothing to free; if a node is allocated, then it's stored in
the tree where it can later be found and reused. "

my updated understanding is that there is no leak in this case- the
nodes remain valid and reusable, and therefore do not require a
separate fix.

David could you correct me if I am mistaken?

[1]
https://lore.kernel.org/linux-mm/20251123132727.3262731-1-shardul.b@mpiricsoftware.com/

[2]
https://lore.kernel.org/linux-mm/57cbf887-d181-418b-a6c7-9f3eff5d632a@arm.com/

[3]
https://lore.kernel.org/all/Ys1r06szkVi3QEai@casper.infradead.org/

Thanks,
Shardul