linux-kernel - Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap

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Date:   Mon, 17 Jul 2023 16:49:19 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap_range()

On 17/07/2023 16:07, Matthew Wilcox wrote:
> On Mon, Jul 17, 2023 at 03:31:09PM +0100, Ryan Roberts wrote:
>> +/*
>> + * folio_remove_rmap_range - take down pte mappings from a range of pages
>> + * belonging to a folio. All pages are accounted as small pages.
>> + * @folio:	folio that all pages belong to
>> + * @page:       first page in range to remove mapping from
>> + * @nr:		number of pages in range to remove mapping from
>> + * @vma:        the vm area from which the mapping is removed
>> + *
>> + * The caller needs to hold the pte lock.
>> + */
> 
> This could stand a little reworking.  How about this?
> 
> /**
>  * folio_remove_rmap_range - Take down PTE mappings from a range of pages.
>  * @folio: Folio containing all pages in range.
>  * @page: First page in range to unmap.
>  * @nr: Number of pages to unmap.
>  * @vma: The VM area containing the range.
>  *
>  * All pages in the range must belong to the same VMA & folio.  They
>  * must be mapped with PTEs, not a PMD.
>  *
>  * Context: Caller holds the pte lock.
>  */

LGTM! thanks.

> 
>> +void folio_remove_rmap_range(struct folio *folio, struct page *page,
>> +					int nr, struct vm_area_struct *vma)
>> +{
>> +	atomic_t *mapped = &folio->_nr_pages_mapped;
>> +	int nr_unmapped = 0;
>> +	int nr_mapped;
>> +	bool last;
>> +	enum node_stat_item idx;
>> +
>> +	if (unlikely(folio_test_hugetlb(folio))) {
>> +		VM_WARN_ON_FOLIO(1, folio);
>> +		return;
>> +	}
>> +
>> +	if (!folio_test_large(folio)) {
>> +		/* Is this the page's last map to be removed? */
>> +		last = atomic_add_negative(-1, &page->_mapcount);
>> +		nr_unmapped = last;
>> +	} else {
>> +		for (; nr != 0; nr--, page++) {
>> +			/* Is this the page's last map to be removed? */
>> +			last = atomic_add_negative(-1, &page->_mapcount);
>> +			if (last) {
>> +				/* Page still mapped if folio mapped entirely */
>> +				nr_mapped = atomic_dec_return_relaxed(mapped);
> 
> We're still doing one atomic op per page on the folio's nr_pages_mapped
> ... is it possible to batch this and use atomic_sub_return_relaxed()?

Good spot, something like this:

	} else {
		for (; nr != 0; nr--, page++) {
			/* Is this the page's last map to be removed? */
			last = atomic_add_negative(-1, &page->_mapcount);
			if (last)
				nr_unmapped++;
		}

		/* Pages still mapped if folio mapped entirely */
		nr_mapped = atomic_sub_return_relaxed(nr_unmapped, mapped);
		if (nr_mapped >= COMPOUND_MAPPED)
			nr_unmapped = 0;
	}