Date: Sun, 20 Sep 2020 15:21:31 +0000 From: Rick Macklem <rmacklem@uoguelph.ca> To: Alan Somers <asomers@freebsd.org>, Konstantin Belousov <kostikbel@gmail.com> Cc: "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r365643 - head/bin/cp Message-ID: <YTBPR01MB3966BD35AAA1D4D7E183FE2BDD3D0@YTBPR01MB3966.CANPRD01.PROD.OUTLOOK.COM> In-Reply-To: <CAOtMX2gtYznWO9UP7f7iryAgh7ngQ%2B5U4L-V5kNp=M2AtD-sLA@mail.gmail.com> References: <202009112049.08BKnavL032212@repo.freebsd.org> <20200911214327.GY94807@kib.kiev.ua> <YTBPR01MB3966E158BCA0C56C5EE64CA8DD240@YTBPR01MB3966.CANPRD01.PROD.OUTLOOK.COM> <CAOtMX2hLLE1c=qbhSFnoRk2SfAg9sOk0PZtiyvNZ_u39YwGKmA@mail.gmail.com> <YTBPR01MB3966A515F676EB5B938AC007DD3C0@YTBPR01MB3966.CANPRD01.PROD.OUTLOOK.COM> <20200919233232.GC94807@kib.kiev.ua>, <CAOtMX2gtYznWO9UP7f7iryAgh7ngQ%2B5U4L-V5kNp=M2AtD-sLA@mail.gmail.com>
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Alan Somers wrote: >On Sat, Sep 19, 2020 at 5:32 PM Konstantin Belousov <kostikbel@gmail.com<mailto:kostikbel@gmail.com>> wrote: >On Sat, Sep 19, 2020 at 11:18:56PM +0000, Rick Macklem wrote: >> Alan Somers wrote: >> >On Fri, Sep 11, 2020 at 3:52 PM Rick Macklem <rmacklem@uoguelph.ca<mailto:rmacklem@uoguelph.ca><mailto:rmacklem@uoguelph.ca<mailto:rmacklem@uoguelph.ca>>> wrote: >> >Konstantin Belousov wrote: >> >>On Fri, Sep 11, 2020 at 08:49:36PM +0000, Alan Somers wrote: >> >>> Author: asomers >> >>> Date: Fri Sep 11 20:49:36 2020 >> >>> New Revision: 365643 >> >>> URL: https://svnweb.freebsd.org/changeset/base/365643 >> >>> >> >>> Log: >> >>> cp: fall back to read/write if copy_file_range fails >> >>> >> >>> Even though copy_file_range has a file-system agnostic version, it still >> >>> fails on devfs (perhaps because the file descriptor is non-seekable?) In >> >>> that case, fallback to old-fashioned read/write. Fixes >> >>> "cp /dev/null /tmp/null" >> >> >> >>Devices are seekable. >> >> >> >>The reason for EINVAL is that vn_copy_file_range() checks that both in and out >> >>vnodes are VREG. For devfs, they are VCHR. >> > >> >I coded the syscall to the Linux man page, which states that EINVAL is returned >> >if either fd does not refer to a regular file. >> >Having said that, I do not recall testing the VCHR case under Linux. (ie. It might >> >actually work and the man page turns out to be incorrect?) >> > >> >I will test this case under Linux when I get home next week, rick >> I'll admit I haven't tested this in Linux to see if they do return EINVAL. >> >> >Since there's no standard, I think it's fine for us to support devfs if possible. >> 1 - I think this is a good question for a mailing list like freebsd-current@. >> 2 - I see Linux as the de-facto standard these days and consider POSIX no >> longer relevant, but that's just mho. >> 3 - For NFSv4.2, the Copy operation will fail for non-regular files, so if you >> do this, you will need to handle the fall-back to using the generic code. >> (Should be doable, but you need to be aware of this case.) >> >> Having said the above, it is up to the "collective" and not me and, as such, >> I suggest #1, to see whether others think doing a non-Linux compatible >> version makes sense for FreeBSD? > >I believe that allowing devfs nodes for vn_copy_file() is not very good >idea. For /dev/null driver returns EOF, but think about real devices or >even better, /dev/zero that never EOF its output. > >Is vn_copy_file() interruptible ? I think not. So if insane range is >specified, we have unstoppable copier that fills the disk (at best). I think this is a serious problem, but the code could clip the "len" argument at K Mbytes for non-VREG files to avoid it (and document that FreeBSD specific behaviour in the man page). >I can think of good use cases for copy_file_range on a device: > >1) Network block devices. I don't know if the iSCSI, NBD, or Ceph RBD protocols >currently support server-side copies, but it's reasonable that they might. If they >ever do, FreeBSD would need copy_file_range to take advantage. >2) CUSE. I think Linux's CUSE already supports copy_file_range, since a CUSE >device on Linux is basically just a single-file FUSE file system. We might add >support to our CUSE driver someday. >3) zvols. This is the use case that matters the most to me. I have a large amount >of data stored in plain files that I would like to convert to zvols. dd should be able >to do that using copy_file_range. > >In my opinion, the utility of those cases outweighs the risk of a long-running >interruptible syscall. And in any case, it is documented that copy_file_range may >return EINTR. I believe that the only case where EINTR would be returned is for NFS mounts with the "intr" option. The generic code uses vn_rdwr()->VOP_READ()/VOP_WRITE() and I think the behaviour w.r.t. signal handling is the same as read(2)/write(2). Is reducing the number of syscalls really going to speed up the above cases? (I did copy_file_range(2) because the copy could be done locally on the NFSv4.2 server. I didn't intend the generic code to be used over read(2)/write(2) to improve performance.) --> I'd suggest you try benchmarking a pre-patched vs current "cp" to copy regular files (not a NFSv4.2 mount) and see if there really is a significant benefit. I'll admit I would prefer a Linux-compatible syscall and think this should be asked on an open mailing list instead of here. rick -Alanhelp
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