Re: [PATCH 05/14] vrange: Add new vrange(2) system call

["H. Peter Anvin" <hpa@zytor.com>]

On 10/02/2013 05:51 PM, John Stultz wrote:
> From: Minchan Kim <minchan@kernel.org>
> 
> This patch adds new system call sys_vrange.
> 
> NAME
> 	vrange - Mark or unmark range of memory as volatile
> 

vrange() is about as nondescriptive as one can get -- there is exactly
one letter that has any connection with that this does.

> SYNOPSIS
> 	int vrange(unsigned_long start, size_t length, int mode,
> 			 int *purged);
> 
> DESCRIPTION
> 	Applications can use vrange(2) to advise the kernel how it should
> 	handle paging I/O in this VM area.  The idea is to help the kernel
> 	discard pages of vrange instead of reclaiming when memory pressure
> 	happens. It means kernel doesn't discard any pages of vrange if
> 	there is no memory pressure.
> 
> 	mode:
> 	VRANGE_VOLATILE
> 		hint to kernel so VM can discard in vrange pages when
> 		memory pressure happens.
> 	VRANGE_NONVOLATILE
> 		hint to kernel so VM doesn't discard vrange pages
> 		any more.
> 
> 	If user try to access purged memory without VRANGE_NOVOLATILE call,
> 	he can encounter SIGBUS if the page was discarded by kernel.
> 
> 	purged: Pointer to an integer which will return 1 if
> 	mode == VRANGE_NONVOLATILE and any page in the affected range
> 	was purged. If purged returns zero during a mode ==
> 	VRANGE_NONVOLATILE call, it means all of the pages in the range
> 	are intact.

I'm a bit confused about the "purged"

>From an earlier version of the patch:

> - What's different with madvise(DONTNEED)?
> 
>   System call semantic
> 
>   DONTNEED makes sure user always can see zero-fill pages after
>   he calls madvise while vrange can see data or encounter SIGBUS.

This difference doesn't seem to be a huge one.  The other one seems to
be the blocking status of MADV_DONTNEED, which perhaps may be better
handled by adding an option (MADV_LAZY) perhaps?

That way we would have lazy vs. immediate, and zero versus SIGBUS.

I see from the change history of the patch that this was an madvise() at
some point, but was changed into a separate system call at some point,
does anyone remember why that was?  A quick look through my LKML
archives doesn't really make it clear.

	-hpa

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