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Message-Id: <20240905002926.1055-1-21cnbao@gmail.com>
Date: Thu, 5 Sep 2024 12:29:26 +1200
From: Barry Song <21cnbao@...il.com>
To: yosryahmed@...gle.com
Cc: 21cnbao@...il.com,
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 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?
>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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