Re: [PATCH] frontswap: enable call to invalidate area on swapoff

[Seth Jennings <spartacus06@gmail.com>]

On Tue, Oct 08, 2013 at 01:08:53PM -0700, Andrew Morton wrote:
> On Tue, 08 Oct 2013 10:13:20 +0200 Krzysztof Kozlowski <k.kozlowski@samsung.com> wrote:
> 
> > On pon, 2013-10-07 at 15:03 -0700, Andrew Morton wrote:
> > > On Mon, 07 Oct 2013 17:25:41 +0200 Krzysztof Kozlowski <k.kozlowski@samsung.com> wrote:
> > > 
> > > > During swapoff the frontswap_map was NULL-ified before calling
> > > > frontswap_invalidate_area(). However the frontswap_invalidate_area()
> > > > exits early if frontswap_map is NULL. Invalidate was never called during
> > > > swapoff.
> > > > 
> > > > This patch moves frontswap_map_set() in swapoff just after calling
> > > > frontswap_invalidate_area() so outside of locks
> > > > (swap_lock and swap_info_struct->lock). This shouldn't be a problem as
> > > > during swapon the frontswap_map_set() is called also outside of any
> > > > locks.
> > > > 
> > > 
> > > Ahem.  So there's a bunch of code in __frontswap_invalidate_area()
> > > which hasn't ever been executed and nobody noticed it.  So perhaps that
> > > code isn't actually needed?
> > > 
> > > More seriously, this patch looks like it enables code which hasn't been
> > > used or tested before.  How well tested was this?
> > > 
> > > Are there any runtime-visible effects from this change?
> > 
> > I tested zswap on x86 and x86-64 and there was no difference. This is
> > good as there shouldn't be visible anything because swapoff is unusing
> > all pages anyway:
> > 	try_to_unuse(type, false, 0); /* force all pages to be unused */
> > 
> > I haven't tested other frontswap users.
> 
> So is that code in __frontswap_invalidate_area() unneeded?

Yes, to expand on what Bob said, __frontswap_invalidate_area() is still
needed to let any frontswap backend free per-swaptype resources.

__frontswap_invalidate_area() is _not_ for freeing structures associated
with individual swapped out pages since all of the pages should be
brought back into memory by try_to_unuse() before
__frontswap_invalidate_area() is called.

The reason we never noticed this for zswap is that zswap has no
dynamically allocated per-type resources.  In the expected case,
where all of the pages have been drained from zswap,
zswap_frontswap_invalidate_area() is a no-op.

Seth

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