Re: [PATCH] mm: Reuse only-pte-mapped KSM page in do_wp_page()

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 13 Dec 2018 18:29:08 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:

> This patch adds an optimization for KSM pages almost
> in the same way, that we have for ordinary anonymous
> pages. If there is a write fault in a page, which is
> mapped to an only pte, and it is not related to swap
> cache; the page may be reused without copying its
> content.
> 
> [Note, that we do not consider PageSwapCache() pages
>  at least for now, since we don't want to complicate
>  __get_ksm_page(), which has nice optimization based
>  on this (for the migration case). Currenly it is
>  spinning on PageSwapCache() pages, waiting for when
>  they have unfreezed counters (i.e., for the migration
>  finish). But we don't want to make it also spinning
>  on swap cache pages, which we try to reuse, since
>  there is not a very high probability to reuse them.
>  So, for now we do not consider PageSwapCache() pages
>  at all.]
> 
> So, in reuse_ksm_page() we check for 1)PageSwapCache()
> and 2)page_stable_node(), to skip a page, which KSM
> is currently trying to link to stable tree. Then we
> do page_ref_freeze() to prohibit KSM to merge one more
> page into the page, we are reusing. After that, nobody
> can refer to the reusing page: KSM skips !PageSwapCache()
> pages with zero refcount; and the protection against
> of all other participants is the same as for reused
> ordinary anon pages pte lock, page lock and mmap_sem.
> 
> ...
>
> +bool reuse_ksm_page(struct page *page,
> +		    struct vm_area_struct *vma,
> +		    unsigned long address)
> +{
> +	VM_BUG_ON_PAGE(is_zero_pfn(page_to_pfn(page)), page);
> +	VM_BUG_ON_PAGE(!page_mapped(page), page);
> +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> +
> +	if (PageSwapCache(page) || !page_stable_node(page))
> +		return false;
> +	/* Prohibit parallel get_ksm_page() */
> +	if (!page_ref_freeze(page, 1))
> +		return false;
> +
> +	page_move_anon_rmap(page, vma);
> +	page->index = linear_page_index(vma, address);
> +	page_ref_unfreeze(page, 1);
> +
> +	return true;
> +}

Can we avoid those BUG_ON()s?

Something like this:

--- a/mm/ksm.c~mm-reuse-only-pte-mapped-ksm-page-in-do_wp_page-fix
+++ a/mm/ksm.c
@@ -2649,9 +2649,14 @@ bool reuse_ksm_page(struct page *page,
 		    struct vm_area_struct *vma,
 		    unsigned long address)
 {
-	VM_BUG_ON_PAGE(is_zero_pfn(page_to_pfn(page)), page);
-	VM_BUG_ON_PAGE(!page_mapped(page), page);
-	VM_BUG_ON_PAGE(!PageLocked(page), page);
+#ifdef CONFIG_DEBUG_VM
+	if (WARN_ON(is_zero_pfn(page_to_pfn(page))) ||
+			WARN_ON(!page_mapped(page)) ||
+			WARN_ON(!PageLocked(page))) {
+		dump_page(page, "reuse_ksm_page");
+		return false;
+	}
+#endif
 
 	if (PageSwapCache(page) || !page_stable_node(page))
 		return false;

We don't have a VM_WARN_ON_PAGE() and we can't provide one because the
VM_foo() macros don't return a value.  It's irritating and I keep
forgetting why we ended up doing them this way.