Date: Sun, 30 Jun 2019 00:45:58 +0000 From: Rick Macklem <rmacklem@uoguelph.ca> To: Alan Somers <asomers@freebsd.org> Cc: "freebsd-fs@freebsd.org" <freebsd-fs@FreeBSD.org>, Sean Fagan <sef@ixsystems.com> Subject: Re: RFC: What should a copy_file_range(2) syscall do by default? Message-ID: <YTXPR01MB02857724AE08766B45D0A608DDFE0@YTXPR01MB0285.CANPRD01.PROD.OUTLOOK.COM> In-Reply-To: <CAOtMX2j0GPhMQ5E6nuU1f0Qiwr_G7ASAG_btf05fb3yFZp_XRw@mail.gmail.com> References: <YTXPR01MB0285B40A9D9A6BD1DC144A64DDE60@YTXPR01MB0285.CANPRD01.PROD.OUTLOOK.COM>, <CAOtMX2j0GPhMQ5E6nuU1f0Qiwr_G7ASAG_btf05fb3yFZp_XRw@mail.gmail.com>
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Well, asomers@ prefers the current patch, despite its complexity.
sef@ expressed concerns w.r.t. the complexity and it being done in
the kernel. However he did not state a preference for any other specific
variant, such as the ones I suggested.
As such, I am sticking with the current patch unless I hear otherwise from
others.
Thanks for your comments, rick
ps: I felt a top post was reasonable here, as it summarized separate posts
in the thread.
________________________________________
From: Alan Somers <asomers@freebsd.org>
Sent: Saturday, June 22, 2019 1:28:03 PM
To: Rick Macklem
Cc: freebsd-fs@freebsd.org; Sean Fagan
Subject: Re: RFC: What should a copy_file_range(2) syscall do by default?
On Sat, Jun 22, 2019 at 10:02 AM Rick Macklem <rmacklem@uoguelph.ca> wrote:
>
> Hi,
>
> sef@ made this comment on phabricator. I don't believe phabricator is the correct
> place for "big picture" discussions, so I'm posting it here (I'm assuming sef@ doesn't
> mind, since the phabricator comments are public).
> sef@ wrote:
> >This much work in the kernel for what //should// be user-space makes me twitchy... >but there is lots of precedent for it, so I obviously have to get with the times.
> >
> > I've done a quick review of the code; it seems most of the complexity is in the hole->detection. I'm also annoyed that linux used size_t for the amount to copy, when >off_t would have been more appropriate. But not much to do about that now.
> >
> > Having a default implementation means that user-space can't fall back if it's not >supported, and do it better (e.g., parallel I/O). Should we also have a pathconf for >the feature?
> >
> > WRT your question on -fs, I have no objections to this working cross-filesystem, >although I think I might ask to have a flag to fail in that case.
>
> Well, all I am interested in is a system call/VOP call so the NFSv4.2 client can do
> a file copy locally on the NFS server instead of doing Reads/Writes across the wire.
> The current code has gotten fairly complex, so I'll try and ask "how complex" this
> syscall/VOP call should be?
>
> The range of variants I can think of are:
> 0) - Don't do it at all.
> 1) - The syscall could just do a VOP_COPY_FILE_RANGE() and return whatever error
> it returns.
> --> This implies an error return for all file systems for now, with support for
> NFSv4.2mounts being added later (FreeBSD13 hopefully).
This option would require applications or the C library to fallback to
a copy loop. While doable, nothing in userland would be able to
range-lock the file, making the copy loop non-atomic. So the
in-kernel copy is superior.
> 2) - The syscall could fall back on a simple copy loop, but not try to deal with holes.
> --> The Linux man page mentions using copy_file_range(2) in a loop with
> lseek(SEEK_DATA)/lseek(SEEK_HOLE) for sparse files. This suggests that
> the Linux fallback code doesn't try to handle holes.
Same problem as 1. Or if you do the copy loop in-kernel it would
waste CPU time and expand sparse files, which isn't good either.
> 3) - The current patch which tries to handle holes and copy the entire byte range
> in one call.
Definitely the best option, despite its complexity. I would argue
that the complexity calls for a robust test suite, rather than
abandoning the feature.
-Alan
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