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
