Date: Sun, 20 Sep 2020 23:14:55 +0000 From: Rick Macklem <rmacklem@uoguelph.ca> To: Alan Somers <asomers@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: RFC: copy_file_range(3) Message-ID: <YTBPR01MB3966C1D4D10BE836B37955F5DD3D0@YTBPR01MB3966.CANPRD01.PROD.OUTLOOK.COM> In-Reply-To: <CAOtMX2gMYdcx0CUC1Mky3ETFr1JkBbYzn17i11axSW=HRTL7OA@mail.gmail.com> References: <CAOtMX2iFZZpoj%2Bap21rrju4hJoip6ZoyxEiCB8852NeH7DAN0Q@mail.gmail.com> <YTBPR01MB39666188FC89399B0D632FE8DD3D0@YTBPR01MB3966.CANPRD01.PROD.OUTLOOK.COM>, <CAOtMX2gMYdcx0CUC1Mky3ETFr1JkBbYzn17i11axSW=HRTL7OA@mail.gmail.com>
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Alan Somers wrote:=0A=
>On Sun, Sep 20, 2020 at 9:58 AM Rick Macklem <rmacklem@uoguelph.ca<mailto:=
rmacklem@uoguelph.ca>> wrote:=0A=
>>Alan Somers wrote:=0A=
>>>copy_file_range(2) is nifty, but it has a few sharp edges:=0A=
>>>1) Certain file systems don't support it, necessitating a write/read bas=
ed=0A=
>>>fallback=0A=
>>>2) It doesn't handle sparse files as well as SEEK_HOLE/SEEK_DATA=0A=
>>>3) It's slightly tricky to both efficiently deal with holes and also=0A=
>>>promptly respond to signals=0A=
>>>=0A=
>>>These problems aren't terribly hard, but it seems to me like most=0A=
>>>applications that use copy_file_range would share the exact same=0A=
>>>solutions. In particular, I'm thinking about cp(1), dd(1), and=0A=
>>>install(8). Those three could benefit from sharing a userland wrapper t=
hat=0A=
>>>handles the above problems.=0A=
>>>=0A=
>>>Should we add such a wrapper to libc? If so, what should it be called, =
and=0A=
>>>should it be public or just private to /usr/src ?=0A=
>>There has been a discussion on src-committers which I suggested should=0A=
>>be taken to a public mailing list.=0A=
>>=0A=
>>The basic question is...=0A=
>>Whether or not the copy_file_range(2) syscall should be compatible with=
=0A=
>>the Linux one.=0A=
>>When I did the syscall, I tried to make it Linux-compatible, arguing that=
=0A=
>>Linux is now a de-facto standard.=0A=
>>The Linux syscall only works on regular files, which is why Alan's patch =
for=0A=
>>cp required a "fallback to the old way" for VCHR files like /dev/null.=0A=
>>=0A=
>>He is considering a wrapper in libc to provide FreeBSD specific semantics=
,=0A=
>>which I have no problem with, so long as the naming and man page make=0A=
>>it clear that it is not compatible with the Linux syscall.=0A=
>>(Personally, I'd prefer a wrapper in libc to making the actual syscall no=
n-Linux=0A=
>> compatible, but that is just mho.)=0A=
>>=0A=
>>Hopefully this helps clarify what Alan is asking, rick=0A=
>>=0A=
>>I don't think the two questions are equivalent. I think that copy_file_r=
ange(2) >>ought to work on character devices. Separately, even it does, I =
think a userland >>wrapper would still be useful. It would still be able t=
o handle sparse files more >>efficiently than the kernel-based vn_generic_c=
opy_file_range.=0A=
I saw this also stated in your #2 above, but wonder why you think a wrapper=
=0A=
would handle holes more efficiently.=0A=
vn_generic_copy_file_range() does look for holes via SEEK_DATA/SEEK_HOLE=0A=
just like a wrapper would and retains them as far as possible. It also look=
s=0A=
for blocks of all zero bytes for file systems that do not support SEEK_DATA=
/=0A=
SEEK_HOLE (like NFS versions prior to 4.2) and creates holes for these in=
=0A=
the output file.=0A=
--> The only cases that I am aware of where the holes are not retained are:=
=0A=
- When the min holesize for the output file is larger than that of the=
=0A=
input file.=0A=
- When the hole straddles the byte range specified for the syscall.=0A=
(Or when the hole straddles two copy_file_range(2) syscalls, if you=
=0A=
prefer.)=0A=
=0A=
If you are copying the entire file and do not care how long the syscall=0A=
takes (which also implies how long it will take for a termination signal=0A=
like <ctrl>C to be handled), the most efficient usage is to specify=0A=
a "len" argument equal to UINT64_MAX.=0A=
--> This will usually copy the whole file in one gulp, although it is not=
=0A=
guaranteed to copy everything, given the Linux semantics definition=
=0A=
of it (an NFSv4.2 server can simply choose to copy less, for example=
).=0A=
--> This allows the kernel to use whatever block size works efficien=
tly=0A=
and does not require an allocation of a large userspace buffer=
for=0A=
the date, nor that the data be copied to/from userspace.=0A=
=0A=
The problem with doing the whole file in one gulp are:=0A=
- A large file can take quite a while and any signal won't be processed unt=
il=0A=
the gulp is done.=0A=
--> If you wrote a program that allocated a 100Gbyte buffer and then=0A=
copied a file using read(2)/write(2) with a size of 100Gbytes in a =
loop,=0A=
you'd end up with the same result.=0A=
- As kib@ noted, if the input file never reports EOF (as /dev/zero does),=
=0A=
then the "one gulp" wouldn't end until storage is exhausted on the=0A=
output file(s) device and <crtl>C wouldn't stop it (since it is one b=
ig=0A=
syscall).=0A=
--> As such, I suggested that, if the syscall is extended to allow VCH=
R,=0A=
that the "len" argument be clipped at "K Mbytes" for that case t=
o=0A=
avoid filling the storage device before being able to <ctrl>C ou=
t=0A=
of it, for this case.=0A=
I suppose the answer for #3 is...=0A=
- smaller "len" allows for quicker response to signals=0A=
but=0A=
- smaller "len" results in less efficient use of the syscall.=0A=
=0A=
Your patch for "cp" seemed fine, but used a small "len" and, as such,=0A=
made the use of copy_file_range(2) less efficient.=0A=
=0A=
All I see the wrapper dong is handling the VCHR case (if the syscall remain=
s=0A=
as it is now and returns EINVAL to be compatible with Linux) and making=0A=
some rather arbitrary choice w.r.t. how big "len" should be.=0A=
--> Choosing an appropriate "len" might better be left to the specific use=
=0A=
case, I think?=0A=
=0A=
In summary, it's mostly whether VCHR gets handled by the syscall or a=0A=
wrapper?=0A=
=0A=
rick=0A=
=0A=
-Alan=0A=
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