linux-kernel - Re: [RFC PATCH 0/2] remove SWAP_MAP

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Message-ID: <CAKEwX=NWJ43LW-N-qLWnWz-wM65_2Ti-UUQe96PC8LDX9VxF+A@mail.gmail.com>
Date: Wed, 25 Sep 2024 07:24:59 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Barry Song <baohua@...nel.org>
Cc: "Huang, Ying" <ying.huang@...el.com>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org, 
	hannes@...xchg.org, hughd@...gle.com, shakeel.butt@...ux.dev, 
	ryan.roberts@....com, chrisl@...nel.org, david@...hat.com, kasong@...cent.com, 
	willy@...radead.org, viro@...iv.linux.org.uk, chengming.zhou@...ux.dev, 
	linux-mm@...ck.org, kernel-team@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM

On Wed, Sep 25, 2024 at 7:21 AM Nhat Pham <nphamcs@...il.com> wrote:
>
>
> I'm only supporting the case nr > 1, when there is no need to add swap
> continuations :) That's the only current use case right now (shmem) :)

Sorry, I forgot to say - but to fully support a batched variant, we
can do something like this:

>
> 1. Keep the non-batched variant:
>
> int swap_duplicate(swp_entry_t entry)
> {
>     int err = 0;
>
>     while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM)
>         err = add_swap_count_continuation(entry, GFP_ATOMIC);
>     return err;
> }
>
> 2. Implement the batched variant:
>
> int swap_duplicate_nr(swp_entry_t entry, int nr)
> {
>     swp_entry_t cur_entry;
>     int i, err;
>
>     if (nr == 1)
>         return swap_duplicate(entry);
>
>     err = __swap_duplicate(entry, 1, nr);
>     if (err == -ENOMEM) {
>         /* fallback to non-batched version */
>         for (i = 0; i < nr; i++) {
>             cur_entry = (swp_entry_t){entry.val + i};
>             if (swap_duplicate(cur_entry)) {
>                 /* rollback */
>                 while (--i >= 0) {
>                      cur_entry = (swp_entry_t){entry.val + i};
>                      swap_free(cur_entry);
>                 }
missing a "return err;" here. Not my best idea to write (pseudo) code
before caffeine in the morning :)
>             }
>         }
>     }
>    return err;
> }
>
> How does this look? My concern is that there is not really a use for
> the fallback logic. Basically dead code.
>
> I can keep it in if you guys have a use for it soon, but otherwise I
> lean towards just adding a WARN etc. there, or return -ENOMEM, and
> WARN at shmem's callsite (because it cannot get -ENOMEM).