linux-kernel - Re: [PATCH v2 02/25] mm: Optimise find

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200212074105.GE7068@infradead.org>
Date:   Tue, 11 Feb 2020 23:41:05 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/25] mm: Optimise find_subpage for !THP

On Tue, Feb 11, 2020 at 08:18:22PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> 
> If THP is disabled, find_subpage can become a no-op.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  include/linux/pagemap.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 75bdfec49710..0842622cca90 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -340,7 +340,7 @@ static inline struct page *find_subpage(struct page *page, pgoff_t offset)
>  
>  	VM_BUG_ON_PAGE(PageTail(page), page);
>  
> -	return page + (offset & (compound_nr(page) - 1));
> +	return page + (offset & (hpage_nr_pages(page) - 1));
>  }

So just above the quoted code is a

	if (PageHuge(page))
		return page;

So how can we get into a compound page that is not a huge page, but
only if THP is enabled?  (Yes, maybe I'm a little rusty on VM
internals).

Can you add comments describing the use case of this function and why
it does all these checks?  It looks like black magic to me.