Re: [PATCH] get_user_pages shortcut for anonymous pages.

[Andrew Morton <akpm@osdl.org>]

Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
>
>  			struct page *map;
>   			int lookup_write = write;
>   			while (!(map = follow_page(mm, start, lookup_write))) {
>  +				/*
>  +				 * Shortcut for anonymous pages. We don't want
>  +				 * to force the creation of pages tables for
>  +				 * insanly big anonymously mapped areas that
>  +				 * nobody touched so far. This is important
>  +				 * for doing a core dump for these mappings.
>  +				 */
>  +				if (!lookup_write && 
>  +				    (!vma->vm_ops || !vma->vm_ops->nopage)) {
>  +					map = ZERO_PAGE(start);
>  +					break;
>  +				}
>   				spin_unlock(&mm->page_table_lock);

I think this will do the wrong thing if the virtual address refers to an
anon page which is swapped out.

You'd need to teach follow_page() to return one of three values:
page-present, page-not-present-but-used-to-be or
page-not-present-and-never-was.

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