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Message-ID: <CAJD7tkbSe+pb_2uhGz5pYTgi9QBzROrfxNM+4qs=JswON31QTA@mail.gmail.com>
Date: Thu, 5 Sep 2024 00:38:43 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, chengming.zhou@...ux.dev, david@...hat.com,
hanchuanhua@...o.com, hannes@...xchg.org, hughd@...gle.com,
kernel-team@...a.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
nphamcs@...il.com, shakeel.butt@...ux.dev, usamaarif642@...il.com,
willy@...radead.org, ying.huang@...el.com, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v4 1/2] mm: store zero pages to be swapped out in a bitmap
On Wed, Sep 4, 2024 at 5:29 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Thu, Sep 5, 2024 at 11:57 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > [..]
> > > well. i feel i have a much cheaper way to implement this, which
> > > can entirely iteration even in your original code:
> > >
> > > +/*
> > > + * Return the number of entries which are zero-filled according to
> > > + * swap_info_struct->zeromap. It isn't precise if the return value
> > > + * is 1 for nr > 1. In this case, it means entries have inconsistent
> > > + * zeromap.
> > > + */
> > > +static inline unsigned int swap_zeromap_entries_count(swp_entry_t
> > > entry, int nr)
> >
> > FWIW I am not really a fan of the count() function not returning an
> > actual count. I think an enum with three states is more appropriate
> > here, and renaming the function to swap_zeromap_entries_check() or
> > similar.
> >
>
> Make sense to me, what about the below?
LGTM from a high level, but I didn't look closely at the bitmap
interface usage. I am specifically unsure if we can pass 'end' as the
size, but it seems like it should be fine. I will let Usama take a
look.
Thanks!
>
> From 24228a1e8426b8b05711a5efcfaae70abeb012c4 Mon Sep 17 00:00:00 2001
> From: Barry Song <v-songbaohua@...o.com>
> Date: Thu, 5 Sep 2024 11:56:03 +1200
> Subject: [PATCH] mm: fix handling zero for large folios with partial zeromap
>
> There could be a corner case where the first entry is non-zeromap,
> but a subsequent entry is zeromap. In this case, we should not
> return false.
>
> Additionally, the iteration of test_bit() is unnecessary and
> can be replaced with bitmap operations, which are more efficient.
>
> Since swap_read_folio() can't handle reading a large folio that's
> partially zeromap and partially non-zeromap, we've moved the code
> to mm/swap.h so that others, like those working on swap-in, can
> access it.
>
> Fixes: 0ca0c24e3211 ("mm: store zero pages to be swapped out in a bitmap")
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
> mm/page_io.c | 27 ++++-----------------------
> mm/swap.h | 29 +++++++++++++++++++++++++++++
> 2 files changed, 33 insertions(+), 23 deletions(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 4bc77d1c6bfa..46907c9dd20b 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -226,26 +226,6 @@ static void swap_zeromap_folio_clear(struct folio *folio)
> }
> }
>
> -/*
> - * Return the index of the first subpage which is not zero-filled
> - * according to swap_info_struct->zeromap.
> - * If all pages are zero-filled according to zeromap, it will return
> - * folio_nr_pages(folio).
> - */
> -static unsigned int swap_zeromap_folio_test(struct folio *folio)
> -{
> - struct swap_info_struct *sis = swp_swap_info(folio->swap);
> - swp_entry_t entry;
> - unsigned int i;
> -
> - for (i = 0; i < folio_nr_pages(folio); i++) {
> - entry = page_swap_entry(folio_page(folio, i));
> - if (!test_bit(swp_offset(entry), sis->zeromap))
> - return i;
> - }
> - return i;
> -}
> -
> /*
> * We may have stale swap cache pages in memory: notice
> * them here and get rid of the unnecessary final write.
> @@ -524,9 +504,10 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>
> static bool swap_read_folio_zeromap(struct folio *folio)
> {
> - unsigned int idx = swap_zeromap_folio_test(folio);
> + unsigned int nr_pages = folio_nr_pages(folio);
> + zeromap_stat_t stat = swap_zeromap_entries_check(folio->swap, nr_pages);
>
> - if (idx == 0)
> + if (stat == SWAP_ZEROMAP_NON)
> return false;
>
> /*
> @@ -534,7 +515,7 @@ static bool swap_read_folio_zeromap(struct folio *folio)
> * currently handled. Return true without marking the folio uptodate so
> * that an IO error is emitted (e.g. do_swap_page() will sigbus).
> */
> - if (WARN_ON_ONCE(idx < folio_nr_pages(folio)))
> + if (WARN_ON_ONCE(stat == SWAP_ZEROMAP_PARTIAL))
> return true;
>
> folio_zero_range(folio, 0, folio_size(folio));
> diff --git a/mm/swap.h b/mm/swap.h
> index f8711ff82f84..f8e3fa061c1d 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -4,6 +4,12 @@
>
> struct mempolicy;
>
> +typedef enum {
> + SWAP_ZEROMAP_NON,
> + SWAP_ZEROMAP_FULL,
> + SWAP_ZEROMAP_PARTIAL
> +} zeromap_stat_t;
> +
> #ifdef CONFIG_SWAP
> #include <linux/swapops.h> /* for swp_offset */
> #include <linux/blk_types.h> /* for bio_end_io_t */
> @@ -80,6 +86,24 @@ static inline unsigned int folio_swap_flags(struct folio *folio)
> {
> return swp_swap_info(folio->swap)->flags;
> }
> +
> +/*
> + * Check if nr entries are all zeromap, non-zeromap or partially zeromap
> + */
> +static inline zeromap_stat_t swap_zeromap_entries_check(swp_entry_t entry, int nr)
> +{
> + struct swap_info_struct *sis = swp_swap_info(entry);
> + unsigned long start = swp_offset(entry);
> + unsigned long end = start + nr;
> +
> + if (find_next_bit(sis->zeromap, end, start) == end)
> + return SWAP_ZEROMAP_NON;
> + if (find_next_zero_bit(sis->zeromap, end, start) == end)
> + return SWAP_ZEROMAP_FULL;
> +
> + return SWAP_ZEROMAP_PARTIAL;
> +}
> +
> #else /* CONFIG_SWAP */
> struct swap_iocb;
> static inline void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
> @@ -171,6 +195,11 @@ static inline unsigned int folio_swap_flags(struct folio *folio)
> {
> return 0;
> }
> +
> +static inline zeromap_stat_t swap_zeromap_entries_check(swp_entry_t entry, int nr)
> +{
> + return SWAP_ZEROMAP_NONE;
> +}
> #endif /* CONFIG_SWAP */
>
> #endif /* _MM_SWAP_H */
> --
> 2.34.1
>
>
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