GNU bug report logs - #26311
dd: add support for POSIX_FADV_NOREUSE etc.

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Package: coreutils; Severity: wishlist; Reported by: Paul Eggert <eggert@HIDDEN>; dated Thu, 30 Mar 2017 15:44:01 UTC; Maintainer for coreutils is bug-coreutils@HIDDEN.
Changed bug title to 'dd: add support for POSIX_FADV_NOREUSE etc.' from 'dd support for POSIX_FADV_NOREUSE etc.' Request was from Assaf Gordon <assafgordon@HIDDEN> to control <at> debbugs.gnu.org. Full text available.
Severity set to 'wishlist' from 'normal' Request was from Assaf Gordon <assafgordon@HIDDEN> to control <at> debbugs.gnu.org. Full text available.

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Subject: Re: bug#26311: dd support for POSIX_FADV_NOREUSE etc.
To: Paul Eggert <eggert@HIDDEN>, 26311 <at> debbugs.gnu.org
References: <6a41f4ba-afcb-4565-bf56-80c7e9898dc0@HIDDEN>
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From: =?UTF-8?Q?P=c3=a1draig_Brady?= <P@HIDDEN>
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Date: Thu, 30 Mar 2017 20:02:17 -0700
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On 30/03/17 10:55, Paul Eggert wrote:
> On 03/30/2017 09:30 AM, Pádraig Brady wrote:
>> Yes maybe, though the low level meanings of these flags are
>> a bit confusing and I'm not sure how consistent they are over kernel versions
> 
> Nor I. Still, 'dd' to some extent is supposed to be a low-level program 
> that lets one directly access syscall flags, so following that design 
> philosophy we should just pass the flags through and let users 
> experiment with them.
> 
>>
>>> Dumb question: why does dd iflag=nocache check the return value of
>>> posix_fadvise? The fadvise function ignores the return value and says
>>> why; do these reasons not apply to 'dd'?
>> Well for dd it's a request, which for other utils it's a performance advisement.
>> As stated in the info docs, for dd it's only significant in the special case
>> of dropping cache for the whole file:
>>
>>    $ : | dd iflag=nocache count=0 status=none
>>    dd: failed to discard cache for: 'standard input': Illegal seek
> 
> Yes, and that was what I was asking about. iflag=nocache is just advice 
> which the kernel is not obliged to follow even when the syscall succeeds 
> (and conversely, it's advice which the kernel might decided to follow 
> even when the syscall fails!). So why bother to report the syscall 
> failure in this particular case?
> 
> It's no big deal. More of a philosophical question, really.

Yes I'm not adamant on this behaviour, but my thinking
was with count=0 iflag=nocache, you are _only_ asking
for cache invalidation, so dd has more leeway to error
in this case (even though the only effective error reported
is EPIPE).  With other dd options, you could be combining
the nocaching directive with other operations and so it
would be ok for the nocache advice failure to be ignored.

cheers,
Pádraig





Information forwarded to bug-coreutils@HIDDEN:
bug#26311; Package coreutils. Full text available.

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Subject: Re: bug#26311: dd support for POSIX_FADV_NOREUSE etc.
To: =?UTF-8?Q?P=c3=a1draig_Brady?= <P@HIDDEN>, 26311 <at> debbugs.gnu.org
References: <6a41f4ba-afcb-4565-bf56-80c7e9898dc0@HIDDEN>
 <7cd0342e-308c-a1af-908f-059df3b9b7b7@HIDDEN>
From: Paul Eggert <eggert@HIDDEN>
Organization: UCLA Computer Science Department
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On 03/30/2017 09:30 AM, P=C3=A1draig Brady wrote:
> Yes maybe, though the low level meanings of these flags are
> a bit confusing and I'm not sure how consistent they are over kernel ve=
rsions

Nor I. Still, 'dd' to some extent is supposed to be a low-level program=20
that lets one directly access syscall flags, so following that design=20
philosophy we should just pass the flags through and let users=20
experiment with them.

>
>> Dumb question: why does dd iflag=3Dnocache check the return value of
>> posix_fadvise? The fadvise function ignores the return value and says
>> why; do these reasons not apply to 'dd'?
> Well for dd it's a request, which for other utils it's a performance ad=
visement.
> As stated in the info docs, for dd it's only significant in the special=
 case
> of dropping cache for the whole file:
>
>    $ : | dd iflag=3Dnocache count=3D0 status=3Dnone
>    dd: failed to discard cache for: 'standard input': Illegal seek

Yes, and that was what I was asking about. iflag=3Dnocache is just advice=
=20
which the kernel is not obliged to follow even when the syscall succeeds=20
(and conversely, it's advice which the kernel might decided to follow=20
even when the syscall fails!). So why bother to report the syscall=20
failure in this particular case?

It's no big deal. More of a philosophical question, really.





Information forwarded to bug-coreutils@HIDDEN:
bug#26311; Package coreutils. Full text available.

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Subject: Re: bug#26311: dd support for POSIX_FADV_NOREUSE etc.
To: Paul Eggert <eggert@HIDDEN>, 26311 <at> debbugs.gnu.org
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On 30/03/17 08:42, Paul Eggert wrote:
> The 'dd' command has some complicated logic for iflag=nocache and 
> oflag=nocache, which I don't entirely understand but expect is in there 
> for good reason. I suggest that 'dd' also have simpler logic for direct 
> usage of posix_fadvise options. For example, 'dd iflag=noreuse' could 
> cause it to call fadvise (STDIN_FILENO, FADVISE_NOREUSE). I suppose 
> 'sequential' should be the default.

Yes maybe, though the low level meanings of these flags are
a bit confusing and I'm not sure how consistent they are over kernel versions,
hence why a higher level "nocache" item was chosen initially.
I have some notes on these settings at:
https://github.com/coreutils/coreutils/blob/c7bcea1b/src/sort.c#L894-L939

> Dumb question: why does dd iflag=nocache check the return value of 
> posix_fadvise? The fadvise function ignores the return value and says 
> why; do these reasons not apply to 'dd'?

Well for dd it's a request, which for other utils it's a performance advisement.
As stated in the info docs, for dd it's only significant in the special case
of dropping cache for the whole file:

  $ : | dd iflag=nocache count=0 status=none
  dd: failed to discard cache for: 'standard input': Illegal seek

cheers,
Pádraig




Information forwarded to bug-coreutils@HIDDEN:
bug#26311; Package coreutils. Full text available.

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From: Paul Eggert <eggert@HIDDEN>
Subject: dd support for POSIX_FADV_NOREUSE etc.
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The 'dd' command has some complicated logic for iflag=nocache and 
oflag=nocache, which I don't entirely understand but expect is in there 
for good reason. I suggest that 'dd' also have simpler logic for direct 
usage of posix_fadvise options. For example, 'dd iflag=noreuse' could 
cause it to call fadvise (STDIN_FILENO, FADVISE_NOREUSE). I suppose 
'sequential' should be the default.

Dumb question: why does dd iflag=nocache check the return value of 
posix_fadvise? The fadvise function ignores the return value and says 
why; do these reasons not apply to 'dd'?





Acknowledgement sent to Paul Eggert <eggert@HIDDEN>:
New bug report received and forwarded. Copy sent to bug-coreutils@HIDDEN. Full text available.
Report forwarded to bug-coreutils@HIDDEN:
bug#26311; Package coreutils. Full text available.
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