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

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Message-ID: <87y3ejh8ax.fsf@yhuang-dev.intel.com>
Date:   Tue, 10 Jul 2018 15:13:58 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Shaohua Li" <shli@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH -mm -v4 05/21] mm, THP, swap: Support PMD swap mapping in free_swap_and_cache()/swap_free()

Dave Hansen <dave.hansen@...ux.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