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

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Message-ID: <ZLVZTupQXt7pAqt8@casper.infradead.org>
Date:   Mon, 17 Jul 2023 16:07:58 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ryan Roberts <ryan.roberts@....com>
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 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.
 */

> +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()?