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From:   Frank Hofmann <fhofmann@cloudflare.com>
Date:   Tue, 18 Oct 2022 09:48:15 +0100
Message-ID: <CABEBQimNm_Ba6j=2cAc8Gw68K_8nVccrT=10LsSkMnTMbo9s3g@mail.gmail.com>
Subject: Re: [PATCH v2] proc: report open files as size in stat() for /proc/pid/fd
To:     David Laight <David.Laight@aculab.com>
Cc:     Ivan Babrou <ivan@cloudflare.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kernel-team@cloudflare.com" <kernel-team@cloudflare.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "Theodore Ts'o" <tytso@mit.edu>, Jonathan Corbet <corbet@lwn.net>,
        David Hildenbrand <david@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>,
        Mike Rapoport <rppt@kernel.org>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Kalesh Singh <kaleshsingh@google.com>
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On Tue, Oct 18, 2022 at 9:25 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Frank Hofmann
> > Sent: 18 October 2022 09:13
> >
> > On Tue, Oct 18, 2022 at 6:02 AM Ivan Babrou <ivan@cloudflare.com> wrote:
> > >
> > > On Mon, Oct 17, 2022 at 6:47 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > > > +
> > > > > +             fdt = files_fdtable(p->files);
> > > > > +             size = fdt->max_fds;
> > > > > +
> > > > > +             for (i = size / BITS_PER_LONG; i > 0;)
> > > > > +                     open_fds += hweight64(fdt->open_fds[--i]);
> > > >
> > > > Could BITMAP_WEIGHT() or __bitmap_weight() or bitmap_weight() be used here?
> > >
> > > That's a great suggestion. I tested it with bitmap_weight() and it
> > > looks much cleaner while providing the same result.
> > >
> > > I just sent the v3 with this suggestion applied.
> >
> > +1 from me on using bitmap_weight() - good spotting that.
>
> Does that have the optimisations for the value being 0, ~0u
> or 2**n-1 all of which are likely for the fd table.
> (Especially if there is no 'popcnt' instruction.)
>
>         David

bitmap_weight() uses hweight_*() under the hood, which then falls
through to platform-specific popcnt where available.
Re, lib/bitmap.c and arch/.../asm/bitops.h or arch/.../hweight.S, for
the impl details.

FrankH.

>
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