linux-kernel - Re: [PATCH v2 02/19] mm, swap: split swap cache preparation loop into a standalone helper

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Message-ID: <CAMgjq7AKWEqzHtMFWcKPCxWtc7jE1j9qGOQ3izsxRef9FCd_+Q@mail.gmail.com>
Date: Mon, 17 Nov 2025 18:04:12 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Chris Li <chrisl@...nel.org>, Nhat Pham <nphamcs@...il.com>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, David Hildenbrand <david@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Youngjun Park <youngjun.park@....com>, 
	Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Ying Huang <ying.huang@...ux.alibaba.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/19] mm, swap: split swap cache preparation loop into
 a standalone helper

On Mon, Nov 17, 2025 at 4:27 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Mon, Nov 17, 2025 at 2:12 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > To prepare for the removal of swap cache bypass swapin, introduce a new
> > helper that accepts an allocated and charged fresh folio, prepares the
> > folio, the swap map, and then adds the folio to the swap cache.
> >
> > This doesn't change how swap cache works yet, we are still depending on
> > the SWAP_HAS_CACHE in the swap map for synchronization. But all
> > synchronization hacks are now all in this single helper.
> >
> > No feature change.
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
>
> Overall,
> Reviewed-by: Barry Song <baohua@...nel.org>
> with a few minor nits.
>
> > ---
> >  mm/swap_state.c | 197 +++++++++++++++++++++++++++++++-------------------------
> >  1 file changed, 109 insertions(+), 88 deletions(-)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 08252eaef32f..7b93704fcbe7 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -402,6 +402,97 @@ void swap_update_readahead(struct folio *folio, struct vm_area_struct *vma,
> >         }
> >  }
> >
> > +/**
> > + * __swap_cache_prepare_and_add - Prepare the folio and add it to swap cache.
> > + * @entry: swap entry to be bound to the folio.
> > + * @folio: folio to be added.
> > + * @gfp: memory allocation flags for charge, can be 0 if @charged if true.
> > + * @charged: if the folio is already charged.
> > + * @skip_if_exists: if the slot is in a cached state, return NULL.
> > + *                  This is an old workaround that will be removed shortly.
> > + *
> > + * Update the swap_map and add folio as swap cache, typically before swapin.
> > + * All swap slots covered by the folio must have a non-zero swap count.
> > + *
> > + * Context: Caller must protect the swap device with reference count or locks.
> > + * Return: Returns the folio being added on success. Returns the existing
> > + * folio if @entry is cached. Returns NULL if raced with swapin or swapoff.
>
> Not a native speaker, but I wonder if adding *already* would make the
> comment clearer, for example: "Returns the existing folio if @entry is
> already cached."

Agree, that sounds better, thanks!

>
> BTW, returning NULL may not be the best option. Returning an ERR_PTR
> seems more specific?

I also don't like the idea of returning NULL, but all existing callers
are all using this convention already. To not introduce too much
noise, this maybe better be cleaned up / changed after this series I
think.