Re: RFC: Remove swap file support

Re: RFC: Remove swap file support

[Eric W. Biederman]

Jeff Garzik <jgarzik@mandrakesoft.com> writes:

> Since you can make any file into a block device using loop,
> is there any value to supporting swap files in 2.5?
> 
> swap files seem like a special case that is no longer necessary...

Yes, and no.  I'd say what we need to do is update rw_swap_page to
use the address space functions directly.  With block devices and
files going through the page cache in 2.5 that should remove any
special cases cleanly.

In 2.4 the swap code really hasn't been updated, the old code has only
been patched enough to work on 2.4.  This adds layers of work that we
really don't need to be doing.  Removing the extra redirection has the
potential to give a small performance boost to swapping.

The case to watch out for are deadlocks doing things like using
swapfiles on an NFS mount.  As you point out we can already do this
with the loop back devices so it isn't really a special case.  The
only new case I can see working are swapfiles with holes in them, or
swapfiles that do automatic compression.  I doubt those cases are
significant improvements but it looks like they will fall out
naturally. 

Eric
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Re: RFC: Remove swap file support

[Chris Wedgwood]

On Sat, Jul 14, 2001 at 12:07:38AM -0600, Eric W. Biederman wrote:

    Yes, and no.  I'd say what we need to do is update rw_swap_page to
    use the address space functions directly.  With block devices and
    files going through the page cache in 2.5 that should remove any
    special cases cleanly.

Will block devices go through the page cache in 2.5.x?

I had hoped they would, that any block devices would just be
page-cache views of underlying character devices, thus allowing us to
remove the buffer-cache and the /dev/raw stuff.



  --cw
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Re: RFC: Remove swap file support

[Eric W. Biederman]

Chris Wedgwood <cw@f00f.org> writes:

> On Sat, Jul 14, 2001 at 12:07:38AM -0600, Eric W. Biederman wrote:
> 
>     Yes, and no.  I'd say what we need to do is update rw_swap_page to
>     use the address space functions directly.  With block devices and
>     files going through the page cache in 2.5 that should remove any
>     special cases cleanly.
> 
> Will block devices go through the page cache in 2.5.x?
> 
> I had hoped they would, that any block devices would just be
> page-cache views of underlying character devices, thus allowing us to
> remove the buffer-cache and the /dev/raw stuff.

<orcale>
Block devices will go through the page cache in 2.5.  It will take a
while for the buffer cache to go away completely, but it is there for
the code paths that haven't been updated.  Buffer heads will stay.

The /dev/raw stuff is for those users that don't want to the kernel to
cache their data and will continue to exist in some form.
</oracle>

I can't see how any device that doesn't support read or writing just a
byte can be a character device.

Eric
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Re: RFC: Remove swap file support

[Chris Wedgwood]

On Sat, Jul 14, 2001 at 11:35:52AM -0600, Eric W. Biederman wrote:

    I can't see how any device that doesn't support read or writing
    just a byte can be a character device.

For requests smaller than the natural block size, you buffer and throw
away... this will surely suck for writing do a hd byte-by-byte though
:)



  --cw
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Re: RFC: Remove swap file support

[Chris Wedgwood]

On Sat, Jul 14, 2001 at 11:35:52AM -0600, Eric W. Biederman wrote:

    I can't see how any device that doesn't support read or writing
    just a byte can be a character device.

Why not... -EINVAL if the read alignment or size is improper...



  --cw
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