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

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Date:   Tue, 8 Mar 2022 18:30:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <namit@...are.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 08.03.22 18:15, Nadav Amit wrote:
> 
> 
>> 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))
> 

Hi Nadav,

> 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?

I don't have a strong opinion, however, I'd prefer keeping it consistent
with existing ways of passing flags.

Personally, I like __bitwise because it just behave the way we're used
to pass flags -- with additional type safety.

Especially once eventually passing many flags (like we do with GFP),
bitfields might turn out rather nasty -- IMHO.


Thanks!

-- 
Thanks,

David / dhildenb