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a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=yJojWOMRYYMA:10 a=GoEa3M9JfhUA:10 a=Z4Rwk6OoAAAA:8 a=yPCof4ZbAAAA:8 a=x7bPxUSWbP2r8J47y-YA:9 a=HkZW87K1Qel5hWWM3VKY:22 cc=ntf awl=host:13614 X-Proofpoint-GUID: j4dTs6n7AXrdPi_qeFgsS2vG6vow7ixV X-Proofpoint-ORIG-GUID: j4dTs6n7AXrdPi_qeFgsS2vG6vow7ixV X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTE2MDIwMyBTYWx0ZWRfX4hbjYUAbsi6l 3N8tBkfPw3A9FWMXATMoVLhEJQXd7ps38smh3SlprM7+CUbEGoc7ZHylkHtieMAHff8DE+Rjfx8 5IYHTkU4qno3yt5dm393Gtas4eAXBp0A60wXevvIVboqGwck8IjYgDacjT7w8BJj8Z+Yb0HGLlg MwiLGwzYy2VSg/KqsTZQiaHp9XUOMC9pvJjCC+3hqlMEyshk7r6PE/zE5e7m+eRFbv6kW8HZ+Qi 82+CQIgg9hu5/j23c2Df2EEASgmqGvknIfY/pzwrYji/ZzYfN6tEW5ST9r2kpj6d37ajkPVp4HX YsPsZXr3XK/0yVT2iZrNtclBqLRd4ZQGGTysJcwKKAD/O/uCpRbGVorcmDiwC6DT50jqB0jTl7J ML+2TAqdoLVnhpYTfnpQHBqj4zuy0g== [ Added Paul McKenney. ] 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