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 18:24:01 +0000
From:   Nadav Amit <namit@...are.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        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 10:09 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Tue, Mar 8, 2022 at 9:15 AM Nadav Amit <namit@...are.com> wrote:
>> 
>> 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.”
> 
> Yeah, even not that old versions had serious issues, iirc.
> 
> Bitfields can look nice, but they have some _serious_ syntax issues.
> In particular, they are nice when you want to *test* one single field
> (ie bit in this case), but basically atrociously bad in almost all
> other circumstances.
> 
> For example, passing a bitfield aggregate as an argument is just
> crazy. Oh, you can do it, with syntax like
> 
>    (struct type) { .field1 = 1, .field3 = 1 }
> 
> as the argument but when you say "much easier to read" I laugh in your
> face and call your mother a hamster.
> 
> And that's ignoring all the issues when you want to combine two
> bitfields. You can't do it. There is nothing like the binary "or"
> operator. Again, it's easy to modify *one* field, but taking two
> bitfields and merging them? Not going to happen.
> 
> So no. Bitfields have their place, but they are close to useless as
> "flags" type things that get passed around as arguments, unless you
> have very very specific and limited use.

I see your point regarding passing an arg. The or’ing of bitfields
can easily be resolved, unless I am missing something, with a union
that holds the aggregate value and an anonymous struct that holds
the individual flags.

At the time, I thought that bitfields are much better fit for cpuid
fields (which are not just flags).

Anyhow, I will refrain from using bitfields for flags, if only for
the sake of my mother. :)