linux-kernel - Re: [PATCH v1 05/15] mm/rmap: convert RMAP flags to a proper distinct rmap

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: <D7AB3A84-3FFB-4E8D-9E68-BF721CC4C6FC@vmware.com>
Date:   Tue, 8 Mar 2022 17:15:36 +0000
From:   Nadav Amit <namit@...are.com>
To:     David Hildenbrand <david@...hat.com>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v1 05/15] mm/rmap: convert RMAP flags to a proper distinct
 rmap_t type



> On Mar 8, 2022, at 6:14 AM, David Hildenbrand <david@...hat.com> wrote:
> 
> We want to pass the flags to more than one anon rmap function, getting
> rid of special "do_page_add_anon_rmap()". So let's pass around a distinct
> __bitwise type and refine documentation.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
> include/linux/rmap.h | 22 ++++++++++++++++++----
> mm/memory.c          |  6 +++---
> mm/rmap.c            |  7 ++++---
> 3 files changed, 25 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 92c3585b8c6a..49f6b208938c 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -158,9 +158,23 @@ static inline void anon_vma_merge(struct vm_area_struct *vma,
> 
> struct anon_vma *page_get_anon_vma(struct page *page);
> 
> -/* bitflags for do_page_add_anon_rmap() */
> -#define RMAP_EXCLUSIVE 0x01
> -#define RMAP_COMPOUND 0x02
> +/* RMAP flags, currently only relevant for some anon rmap operations. */
> +typedef int __bitwise rmap_t;
> +
> +/*
> + * No special request: if the page is a subpage of a compound page, it is
> + * mapped via a PTE. The mapped (sub)page is possibly shared between processes.
> + */
> +#define RMAP_NONE		((__force rmap_t)0)
> +
> +/* The (sub)page is exclusive to a single process. */
> +#define RMAP_EXCLUSIVE		((__force rmap_t)BIT(0))
> +
> +/*
> + * The compound page is not mapped via PTEs, but instead via a single PMD and
> + * should be accounted accordingly.
> + */
> +#define RMAP_COMPOUND		((__force rmap_t)BIT(1))

I was once shouted at for a similar suggestion, but I am going to try
once more… If you already define a new type, why not to use bitfields?

It would be much easier to read. The last time I made such a suggestion,
Ingo said "I personally like bitfields in theory … [but] older versions of
GCC did a really poor job of optimizing them.” At the time (2014), I looked
at GCC-4.4 and GCC-4.8 and there were some differences in the quality of
the generated code. Is it still the case?