Re: [PATCH -mm -v4 05/21] mm, THP, swap: Support PMD swap mapping in free_swap_and_cache()/swap_free()

Huang, Ying
2018-07-10T07:14:06Z

Dave Hansen <dave.hansen@linux.intel.com> writes:

> I'm seeing a pattern here.
>
> old code:
>
> foo()
> {
> do_swap_something()
> }
>
> new code:
>
> foo(bool cluster)
> {
> if (cluster)
> do_swap_cluster_something();
> else
> do_swap_something();
> }
>
> That make me fear that we have:
> 1. Created a new, wholly untested code path
> 2. Created two places to patch bugs
> 3. Are not reusing code when possible
>
> The code non-resuse was, and continues to be, IMNHO, one of the largest
> sources of bugs with the original THP implementation. It might be
> infeasible to do here, but let's at least give it as much of a go as we can.

I totally agree that we should unify the code path for huge and normal
page/swap if possible. One concern is code size for !CONFIG_THP_SWAP.
The original method is good for that. The new method may introduce some
huge swap related code that is hard to be eliminated for
!CONFIG_THP_SWAP. Andrew Morton pointed this out for the patchset of
the first step of the THP swap optimization.

This may be mitigated at least partly via,

`
#ifdef CONFIG_THP_SWAP
#define nr_swap_entries(nr) (nr)
#else
#define nr_swap_entries(nr) 1
#endif

void do_something(swp_entry_t entry, int __nr_entries)
{
int i, nr_entries = nr_swap_entries(__nr_entries);

if (nr_entries = SWAPFILE_CLUSTER)
; /* huge swap specific */
else
; /* normal swap specific */

for (i = 0; i < nr_entries; i++) {
; /* do something for each entry */
}

/* ... */
}
`

and rely on compiler to do the dirty work for us if possible.

Hi, Andrew,

What do you think about this?

> Can I ask that you take another round through this set and:
>
> 1. Consolidate code refactoring into separate patches

Sure.

> 2. Add comments to code, and avoid doing it solely in changelogs

Sure.

> 3. Make an effort to share more code between the old code and new
> code. Where code can not be shared, call that out in the changelog.

Will do that if we resolve the code size concern.

> This is a *really* hard-to-review set at the moment. Doing those things
> will make it much easier to review and hopefully give us more
> maintainable code going forward.
>
> My apologies for not having done this review sooner.

Thanks a lot for your comments!

Best Regards,
Huang, Ying