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Re: Interface for the new fallocate() system call

To: "Amit K. Arora" <aarora@xxxxxxxxxxxxxxxxxx>
Subject: Re: Interface for the new fallocate() system call
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Thu, 29 Mar 2007 10:10:10 -0700
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, suparna@xxxxxxxxxx, cmm@xxxxxxxxxx
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On Thu, 29 Mar 2007 17:21:26 +0530 "Amit K. Arora" <aarora@xxxxxxxxxxxxxxxxxx> 

> Hello,
> We need to come up with the best possible layout of arguments for the
> fallocate() system call. Various architectures have different
> requirements for how the arguments should look like. Since the mail
> chain has become huge, here is the summary of various inputs received
> so far.
> Platform: s390
> --------------
> s390 prefers following layout:
>    int fallocate(int fd, loff_t offset, loff_t len, int mode)
> For details on why and how "int, int, loff_t, loff_t" is a problem on
> s390, please see Heiko's mail on 16th March. Here is the link:
> http://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg133595.html
> Platform: ppc, arm
> ------------------
> ppc (32 bit) has a problem with "int, loff_t, loff_t, int" layout,
> since this will result in a pad between fd and offset, making seven
> arguments total - which is not supported by ppc32. It supports only
> 6 arguments. Thus the desired layout by ppc32 is:
>    int fallocate(int fd, int mode, loff_t offset, loff_t len)
> Even ARM prefers above kind of layout. For details please see the
> definition of sys_arm_sync_file_range().

This is a clean-looking option.  Can s390 be changed to support seven-arg

> Option of loff_t => high u32 + low u32
> --------------------------------------
> Matthew and Russell have suggested another option of breaking each
> "loff_t" into two "u32"s. This will result in 6 arguments in total.
> Following think that this is a good alternative:
> Matthew Wilcox, Russell King, Heiko Carstens
> Following do not like this idea:
> Chris Wedgwood

It's a bit weird-looking, but the six-32-bit-args approach is simple
enought to understand and implement.  Presumably the glibc wrapper
would hide that detail from everyone.
> What are your thoughts on this ? What layout should we finalize on ?
> Perhaps, since sync_file_range() system call has similar arguments, we
> can take hint from the challenges faced on implementing it on various
> architectures, and decide.

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