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 KztLVt0f 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Andrew Morton writes: > On Wed, 17 Sep 2025 08:24:15 -0700 Ankur Arora wrote: > >> Change folio_zero_user() to clear contiguous page ranges instead of >> clearing using the current page-at-a-time approach. Exposing the largest >> feasible length can be useful in enabling processors to optimize based >> on extent. > > This patch is something which MM developers might care to take a closer > look at. > >> However, clearing in large chunks can have two problems: >> >> - cache locality when clearing small folios (< MAX_ORDER_NR_PAGES) >> (larger folios don't have any expectation of cache locality). >> >> - preemption latency when clearing large folios. >> >> Handle the first by splitting the clearing in three parts: the >> faulting page and its immediate locality, its left and right >> regions; with the local neighbourhood cleared last. > > Has this optimization been shown to be beneficial? So, this was mostly meant to be defensive. The current code does a rather extensive left-right dance around the faulting page via c6ddfb6c58 ("mm, clear_huge_page: move order algorithm into a separate function") and I wanted to keep the cache hot property for the region closest to the address touched by the user. But, no I haven't run any tests showing that it helps. > If so, are you able to share some measurements? >From some quick kernel builds (with THP) I do see a consistent difference of a few seconds (1% worse) if I remove this optimization. (I'm not sure right now why it is worse -- my expectation was that we would have higher cache misses, but I see pretty similar cache numbers.) But let me do a more careful test and report back. > If not, maybe it should be removed? > >> ... >> >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -7021,40 +7021,80 @@ static inline int process_huge_page( >> return 0; >> } >> >> -static void clear_gigantic_page(struct folio *folio, unsigned long addr_hint, >> - unsigned int nr_pages) >> +/* >> + * Clear contiguous pages chunking them up when running under >> + * non-preemptible models. >> + */ >> +static void clear_contig_highpages(struct page *page, unsigned long addr, >> + unsigned int npages) > > Called "_highpages" because it wraps clear_user_highpages(). It really > should be called clear_contig_user_highpages() ;) (Not serious) Or maybe clear_user_contig_highpages(), so when we get rid of HUGEMEM, the _highpages could just be chopped off :D. >> { >> - unsigned long addr = ALIGN_DOWN(addr_hint, folio_size(folio)); >> - int i; >> + unsigned int i, count, unit; >> >> - might_sleep(); >> - for (i = 0; i < nr_pages; i++) { >> + unit = preempt_model_preemptible() ? npages : PAGE_CONTIG_NR; > > Almost nothing uses preempt_model_preemptible() and I'm not usefully > familiar with it. Will this check avoid all softlockup/rcu/etc > detections in all situations (ie, configs)? IMO, yes. The code invoked under preempt_model_preemptible() will boil down to a single interruptible REP STOSB which might execute over an extent of 1GB (with the last patch). From prior experiments, I know that irqs are able to interrupt this. And, I /think/ that is a sufficient condition for avoiding RCU stalls/softlockups etc. Also, when we were discussing lazy preemption (which Thomas had suggested as a way to handle scenarios like this or long running Xen hypercalls etc) this seemed like a scenario that didn't need any extra handling for CONFIG_PREEMPT. We did need 83b28cfe79 ("rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y") for CONFIG_PREEMPT_LAZY but AFAICS this should be safe. Anyway let me think about your all configs point (though only ones which can have some flavour for hugetlb.) Also, I would like x86 folks opinion on this. And, maybe Paul McKenney just to make sure I'm not missing something on RCU side. Thanks for the comments. -- ankur