RE: [PATCH v3 04/10] mm: frontswap: split out __frontswap_unuse_pages

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Sasha Levin [mailto:levinsasha928@gmail.com]
> Subject: Re: [PATCH v3 04/10] mm: frontswap: split out __frontswap_unuse_pages
> 
> > > +	assert_spin_locked(&swap_lock);
> >
> > Normally, we should use this assertion when we can't find swap_lock is hold or not easily
> > by complicated call depth or unexpected use-case like general function.
> > But I expect this function's caller is very limited, not complicated.
> > Just comment write down isn't enough?
> 
> Is there a reason not to do it though? Debugging a case where this
> function is called without a swaplock and causes corruption won't be
> easy.

I'm not sure of the correct kernel style but I like the fact
that assert_spin_locked both documents the lock requirement and tests
it at runtime.

I don't know the correct kernel syntax but is it possible
to make this code be functional when the kernel "debug"
option is on, but a no-op when "debug" is disabled?
IMHO, that would be the ideal solution.
 
> > > +	for (type = swap_list.head; type >= 0; type = si->next) {
> > > +		si = swap_info[type];
> > > +		si_frontswap_pages = atomic_read(&si->frontswap_pages);
> > > +		if (total_pages_to_unuse < si_frontswap_pages) {
> > > +			pages = pages_to_unuse = total_pages_to_unuse;
> > > +		} else {
> > > +			pages = si_frontswap_pages;
> > > +			pages_to_unuse = 0; /* unuse all */
> > > +		}
> > > +		/* ensure there is enough RAM to fetch pages from frontswap */
> > > +		if (security_vm_enough_memory_mm(current->mm, pages)) {
> > > +			ret = -ENOMEM;
> >
> >
> > Nipick:
> > I am not sure detailed error returning would be good.
> > Caller doesn't matter it now but it can consider it in future.
> > Hmm,
> 
> Is there a reason to avoid returning a meaningful error when it's pretty
> easy?

I'm certainly not an expert on kernel style (as this whole series
of patches demonstrates :-) but I think setting a meaningful
error code is useful documentation and plans for future users
that might use the error code.

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