On Tue, 29 Jul 2025, Dongsheng Yang wrote:
> > > > Just a suggestion for performance improvement: when you hold a
> > > > spinlock a you need to allocate some memory, you must drop the
> > > > spinlock, allocate it with GFP_NOIO, reacquire the spinlock and
> > > > recheck the state. You can improve this logic by allocating with
> > > > mempool_alloc(GFP_NOWAIT) while you hold the spinlock (GFP_NOWAIT
> > > > will not sleep, so it's allowed), and fallback to dropping the
> > > > spinlock only if the GFP_NOWAIT allocation fails.
> > >
> > > Sounds a good suggestion, I will try it in V5.
> >
> > Yes. When you implement it, don't forget to test the code path that drops
> > the spinlock and uses mempool_alloc(GFP_NOIO), so that it doesn't bitrot
> > over time.
>
>
> Sure, the failslab case can cover this path:
>
> https://github.com/DataTravelGuide/dtg-tests/blob/main/pcache.py.data/pcache_failslab.sh
>
>
> Dongsheng
failslab doesn't test mempool allocation failures - see the function
mempool_alloc_noprof:
it tries to allocate using
element = pool->alloc(gfp_temp, pool->pool_data);
if it fails (the failure may be caused by failslab), it goes down this
path
spin_lock_irqsave(&pool->lock, flags);
if (likely(pool->curr_nr)) {
element = remove_element(pool);
spin_unlock_irqrestore(&pool->lock, flags);
/* paired with rmb in mempool_free(), read comment there */
smp_wmb();
/*
* Update the allocation stack trace as this is more useful
* for debugging.
*/
kmemleak_update_trace(element);
return element;
}
so, it will not simulate a situation where the mempool is depleted.
It may be worthwhile to extend failslab, so that it makes mempool_alloc
randomly return NULL for non-sleeping allocations (i.e. allocations
without __GFP_DIRECT_RECLAIM) and submit the patch to failslab
maintainers.
Mikulas
