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

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Date:   Tue, 29 Aug 2023 15:02:22 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Matthew Wilcox <willy@...radead.org>
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 25/08/2023 05:09, Matthew Wilcox wrote:
> 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?

Good point. This function is originally a copy/paste of release_pages(), and I
obviously missed this. In fact, looking at it again today, I think I'll factor
out the vast majority into a common helper, since I'm currently duplicating a
whole bunch here.

In practice I think all devmap folios will be small today though? So while I
agree I need to fix this, I think in practice it will currently do the right thing?

> 
>> +				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.

OK, what's it called? This is just copied from release_pages() at the moment.
Happy to use your helper in the refactored common helper.

>