linux-kernel - Re: [PATCH v1 4/4] mm/mmu_gather: Store and process pages in contig ranges

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Date:   Fri, 25 Aug 2023 05:09:19 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Will Deacon <will@...nel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 4/4] mm/mmu_gather: Store and process pages in contig
 ranges

On Thu, Aug 10, 2023 at 11:33:32AM +0100, Ryan Roberts wrote:
> +void folios_put_refs(struct folio_range *folios, int nr)
> +{
> +	int i;
> +	LIST_HEAD(pages_to_free);
> +	struct lruvec *lruvec = NULL;
> +	unsigned long flags = 0;
> +	unsigned int lock_batch;
> +
> +	for (i = 0; i < nr; i++) {
> +		struct folio *folio = page_folio(folios[i].start);
> +		int refs = folios[i].end - folios[i].start;
> +
> +		/*
> +		 * Make sure the IRQ-safe lock-holding time does not get
> +		 * excessive with a continuous string of pages from the
> +		 * same lruvec. The lock is held only if lruvec != NULL.
> +		 */
> +		if (lruvec && ++lock_batch == SWAP_CLUSTER_MAX) {
> +			unlock_page_lruvec_irqrestore(lruvec, flags);
> +			lruvec = NULL;
> +		}
> +
> +		if (is_huge_zero_page(&folio->page))
> +			continue;
> +
> +		if (folio_is_zone_device(folio)) {
> +			if (lruvec) {
> +				unlock_page_lruvec_irqrestore(lruvec, flags);
> +				lruvec = NULL;
> +			}
> +			if (put_devmap_managed_page(&folio->page))
> +				continue;
> +			if (folio_put_testzero(folio))

We're only putting one ref for the zone_device folios?  Surely
this should be ref_sub_and_test like below?

> +				free_zone_device_page(&folio->page);
> +			continue;
> +		}
> +
> +		if (!folio_ref_sub_and_test(folio, refs))
> +			continue;
> +
> +		if (folio_test_large(folio)) {
> +			if (lruvec) {
> +				unlock_page_lruvec_irqrestore(lruvec, flags);
> +				lruvec = NULL;
> +			}
> +			__folio_put_large(folio);
> +			continue;
> +		}
> +
> +		if (folio_test_lru(folio)) {
> +			struct lruvec *prev_lruvec = lruvec;
> +
> +			lruvec = folio_lruvec_relock_irqsave(folio, lruvec,
> +									&flags);
> +			if (prev_lruvec != lruvec)
> +				lock_batch = 0;
> +
> +			lruvec_del_folio(lruvec, folio);
> +			__folio_clear_lru_flags(folio);
> +		}
> +
> +		/*
> +		 * In rare cases, when truncation or holepunching raced with
> +		 * munlock after VM_LOCKED was cleared, Mlocked may still be
> +		 * found set here.  This does not indicate a problem, unless
> +		 * "unevictable_pgs_cleared" appears worryingly large.
> +		 */
> +		if (unlikely(folio_test_mlocked(folio))) {
> +			__folio_clear_mlocked(folio);
> +			zone_stat_sub_folio(folio, NR_MLOCK);
> +			count_vm_event(UNEVICTABLE_PGCLEARED);
> +		}

You'll be glad to know I've factored out a nice little helper for that.