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Date:      Sat, 27 Jul 2019 10:17:10 -0700
From:      Conrad Meyer <cem@freebsd.org>
To:        Rick Macklem <rmacklem@uoguelph.ca>
Cc:        "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org>
Subject:   Re: RFC: should copy_file_range(2) use size_t or off_t for length argument
Message-ID:  <CAG6CVpWdumaF%2BG4yohzS=G1wzrQzrezi7nmbVfPTRPFexqbirw@mail.gmail.com>
In-Reply-To: <YTBPR01MB33122A0A1B3CD9D8EC18A0ECDDC30@YTBPR01MB3312.CANPRD01.PROD.OUTLOOK.COM>
References:  <YTBPR01MB33122A0A1B3CD9D8EC18A0ECDDC30@YTBPR01MB3312.CANPRD01.PROD.OUTLOOK.COM>

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Just my 2=C2=A2: keep the 1:1 Linux compatible interface.  Requiring
programs to loop over N x 2^31 copies of larger files on 32-bit
platforms does not impose significant extra syscall burden on copy
programs over the wider off_t, and it fits the pattern of many
existing synchronous IO APIs (size_t lengths).

I think there is some benefit to matching other OS's non-POSIX
function APIs exactly when we choose to use those same names and
concepts =E2=80=94 ifdef soup is painful.  And developers target Linux firs=
t.

Cheers,
Conrad

On Sat, Jul 27, 2019 at 9:40 AM Rick Macklem <rmacklem@uoguelph.ca> wrote:
>
> Hi,
>
> r350315 implemented a Linux compatible copy_file_range(2) syscall.
> Since Linux used a length argument of size_t and a return argument
> type of ssize_t, I did the same.
>
> Kostik has suggested that making these off_t would allow a full 64bit
> copy be done on 32bit arches.
> Here is the snippet of discussion we have had:
> Kostik Belousov wrote:
> > >Kostik Belousov wrote:
> >> >I sat to write the compat32 shims, and only then noted that len has s=
ize_t
> >> >type.  Why is it size_t and not off_t ?
> > I wrote:
> >> Well, that's what Linux did.
> >>
> >> Also, since it returns ssize_t, it can't do more than SSIZE_MAX
> >> (generally 1/2 of SIZE_T_MAX). Returning ssize_t is also what Linux
> >> does and is consistent with read(2)/write(2).
> >
> >If changing the length argument type to off_t, it is reasonable to chang=
e
> >the return type to off_t as well.  We already have the lseek(2) syscall =
that
> >requires two return registers on 32bit.
> >
> >Note that it is reasonable for read(2) to take length as size_t-typed
> >parameter, because size_t is the type for object sizes. There is no
> >object in user address space for copy_file_range(2) API, so potentially
> >wider off_t is acceptable and is in fact useful there. It is useful on
> >32bit machines where FreeBSD size_t is 32bit, while off_t is 64bit.
>
> So, what do others think?
> (My only concern w.r.t. changing the arguments to off_t is Linux compatib=
ility.)
>
> rick
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