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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Date:   Tue, 10 Jul 2018 07:07:39 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     "Huang, Ying" <ying.huang@...el.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()

On 07/10/2018 12:13 AM, Huang, Ying wrote:
> Dave Hansen <dave.hansen@...ux.intel.com> writes:
>> 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.

I've honestly never heard that as an argument before.  In general, our
.c files implement *full* functionality: the most complex case.  The
headers #ifdef that functionality down because of our .config or
architecture.

The thing that matters here is debugging and reviewing the _complicated_
case, IMNHO.

> 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 */
>         }
> 
>         /* ... */
> }
> `

While that isn't perfect, it's better than the current state of things.

While you are refactoring things, I think you also need to take a good
look at roughly chopping this series in half by finding another stopping
point.  You've done a great job so far of trickling this functionality
in so far, but 21 patches is quite a bit, and the set is only going to
get larger.