Re: [RFC/RFT][PATCH -mm] swsusp: userland interface

["Rafael J. Wysocki" <rjw@sisk.pl>]

Hi,

On Saturday, 14 January 2006 12:29, Pavel Machek wrote:
> > > > > > > It returns the number of pages.  Well, it should be written explicitly,
> > > > > > > so I'll fix that.
> > > > > > 
> > > > > > Please always talk to the kernel in bytes. Pagesize is only a kernel
> > > > > > internal unit. Sth. like off64_t is fine.
> > > > > 
> > > > > These are values returned by the kernel, actually.  Of course I can convert them
> > > > > to bytes before sending to the user space, if that's preferrable.
> > > > > 
> > > > > Pavel, what do you think?
> > > > 
> > > > Bytes, I'd say. It would be nice if preffered image size was in bytes,
> > > > too, for consistency.
> > > 
> > > OK
> > 
> > Having actually tried to do that I see two reasons for keeping the image size
> > in megs.
> > 
> > First, if that was in bytes, I'd have to pass it via a pointer, because
> > unsigned long might overflow on i386.  Then I'd have to use get_user()
> 
> Actually unsigned long is okay. We can't do images > 1.5GB, anyway,
> on i386.

Right.  In fact 1/2 of lowmem is the limit.

> > to read the value.  However, afterwards I'd have to rescale that value
> > to megs for swsusp_shrink_memory().  It's just easier to pass the value
> > in megs using the last argument of ioctl() directly (which is consistent
> > with the /sys/power/image_size thing, BTW).
> 
> Well, I'd be inclined to make image_size in bytes, too. Having
> each ioctl/sys file in different units seems wrong.

I'll add these changes to the userland interface patch, then.  There won't
be too many of them, I think.

> > Second, if that's in bytes, it would suggest that the memory-shrinking
> > mechanism had byte granularity (ie. way off).
> 
> Yep, but it is not that bad.
> 
> > There also is a reason for which SNAPSHOT_AVAIL_SWAP should return
> > the number of pages, IMO.  Namely, if that's in pages, the number is directly
> > comparable with the number of image pages which the suspending
> > utility can read from the image header.  Otherwise it would have to rescale
> > one of these values using PAGE_SIZE, but that's exactly what we'd like
> > to avoid.
> 
> I see.... We could put #bytes into image header (unsigned long) :-).
> 
> Its not too bad one way or another, because uswsusp tools are
> intimately tied to kernel, anyway.

Exactly. :-)

Greetings,
Rafael
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