Re: [PATCH v2 14/19] maple_tree: WARN_ON_ONCE when allocations fail

[Liam R. Howlett (Oracle)]

On 26/06/30 04:02PM, Andrew Morton wrote:
> On Tue, 30 Jun 2026 15:08:38 -0400 "Liam R. Howlett (Oracle)" <liam@xxxxxxxxxxxxx> wrote:
> 
> > Allocations should never fail in the circumstances that are expected to
> > occur.  Add checks in the code to ensure the circumstances are correctly
> > set up by the user and warn if they are not.
> > 
> > Also add a warning on failure to allocate, which should never happen.
> > 
> > ...
> >
> > --- a/lib/maple_tree.c
> > +++ b/lib/maple_tree.c
> > @@ -5720,6 +5720,10 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
> >  	if (likely(mas->node != MA_ERROR(-ENOMEM)))
> >  		return false;
> >  
> > +	/* Allocations can fail, don't do this. */
> > +	WARN_ON_ONCE(!gfpflags_allow_blocking(gfp) &&
> > +		     mt_external_lock(mas->tree));
> > +
> >  	if (gfpflags_allow_blocking(gfp) && !mt_external_lock(mas->tree)) {
> >  		mtree_unlock(mas->tree);
> >  		mas_alloc_nodes(mas, gfp);
> > @@ -5730,9 +5734,12 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
> >  
> >  	/*
> >  	 * Return false on zero forward progress.  Partial allocations are kept
> > -	 * so the retry path will attempt to get the rest.
> > +	 * so the retry path will attempt to get the rest.  The failure should
> > +	 * not happen as we try our best to reclaim.  The user would need an
> > +	 * external lock with a non-blocking gfp in a low memory situation -
> > +	 * which would have triggered the first warning in this function.
> >  	 */
> > -	if (!mas->sheaf && !mas->alloc)
> > +	if (WARN_ON_ONCE(!mas->sheaf && !mas->alloc))
> >  		return false;
> >  
> 
> Can either of these warnings duplicate the effect of !__GFP_NOWARN?

>From what I understand, __GFP_NOWARN is about suppressing warnings on
failures because you handle them at a higher level, but in this case I
am trying to detect incorrect use of the interface.

That is, if you want to handle the failures yourself then you should use
mas_preallocate() that is designed to give status on the allocations.
The following call to mas_store_prealloc() will not need to allocate or
check for nomem.

Thanks,
Liam