2020-02-29 13:17:08

by Wei Yang

Subject: [PATCH] mm/swapfile.c: simplify the scan loop in scan_swap_map_slots()

After commit c60aa176c6de8 ("swapfile: swap allocation cycle if
nonrot"), swap allocation is cyclic. Current approach is done with two
separate loop on the upper and lower half. This looks a little
redundant.

From another point of view, the loop iterates [lowest_bit, highest_bit]
range starting with (offset + 1) but except scan_base. So we can
simplify the loop with condition (next_offset() != scan_base) by
introducing next_offset() which makes sure offset fit in that range
with correct order.

Signed-off-by: Wei Yang <[email protected]>
CC: Hugh Dickins <[email protected]>
---
mm/swapfile.c | 26 +++++++++-----------------
1 file changed, 9 insertions(+), 17 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 95024f9b691a..42c5c2010bfc 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -729,6 +729,14 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
}
}

+static unsigned long next_offset(struct swap_info_struct *si,
+ unsigned long *offset)
+{
+ if (++(*offset) > si->highest_bit)
+ *offset = si->lowest_bit;
+ return *offset;
+}
+
static int scan_swap_map_slots(struct swap_info_struct *si,
unsigned char usage, int nr,
swp_entry_t slots[])
@@ -883,7 +891,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,

scan:
spin_unlock(&si->lock);
- while (++offset <= si->highest_bit) {
+ while (next_offset(si, &offset) != scan_base) {
if (!si->swap_map[offset]) {
spin_lock(&si->lock);
goto checks;
@@ -897,22 +905,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
latency_ration = LATENCY_LIMIT;
}
}
- offset = si->lowest_bit;
- while (offset < scan_base) {
- if (!si->swap_map[offset]) {
- spin_lock(&si->lock);
- goto checks;
- }
- if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
- spin_lock(&si->lock);
- goto checks;
- }
- if (unlikely(--latency_ration < 0)) {
- cond_resched();
- latency_ration = LATENCY_LIMIT;
- }
- offset++;
- }
spin_lock(&si->lock);

no_page:
--
2.23.0

2020-02-29 22:08:24

by Wei Yang

Subject: Re: [PATCH] mm/swapfile.c: simplify the scan loop in scan_swap_map_slots()

On Sat, Feb 29, 2020 at 01:15:37PM +0000, Wei Yang wrote:
>After commit c60aa176c6de8 ("swapfile: swap allocation cycle if
>nonrot"), swap allocation is cyclic. Current approach is done with two
>separate loop on the upper and lower half. This looks a little
>redundant.
>
>>From another point of view, the loop iterates [lowest_bit, highest_bit]
>range starting with (offset + 1) but except scan_base. So we can
>simplify the loop with condition (next_offset() != scan_base) by
>introducing next_offset() which makes sure offset fit in that range
>with correct order.
>
>Signed-off-by: Wei Yang <[email protected]>
>CC: Hugh Dickins <[email protected]>
>---
> mm/swapfile.c | 26 +++++++++-----------------
> 1 file changed, 9 insertions(+), 17 deletions(-)
>
>diff --git a/mm/swapfile.c b/mm/swapfile.c
>index 95024f9b691a..42c5c2010bfc 100644
>--- a/mm/swapfile.c
>+++ b/mm/swapfile.c
>@@ -729,6 +729,14 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
> }
> }
>
>+static unsigned long next_offset(struct swap_info_struct *si,
>+ unsigned long *offset)
>+{
>+ if (++(*offset) > si->highest_bit)
>+ *offset = si->lowest_bit;

Hmm... I found one potential problem here. If someone has eaten the lower
part, (si->lowest_bit > scan_base), we would fall into infinite loop.

Will wait for some comment before sending v2.

>+ return *offset;
>+}
>+
> static int scan_swap_map_slots(struct swap_info_struct *si,
> unsigned char usage, int nr,
> swp_entry_t slots[])
>@@ -883,7 +891,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>
> scan:
> spin_unlock(&si->lock);
>- while (++offset <= si->highest_bit) {
>+ while (next_offset(si, &offset) != scan_base) {
> if (!si->swap_map[offset]) {
> spin_lock(&si->lock);
> goto checks;
>@@ -897,22 +905,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> latency_ration = LATENCY_LIMIT;
> }
> }
>- offset = si->lowest_bit;
>- while (offset < scan_base) {
>- if (!si->swap_map[offset]) {
>- spin_lock(&si->lock);
>- goto checks;
>- }
>- if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
>- spin_lock(&si->lock);
>- goto checks;
>- }
>- if (unlikely(--latency_ration < 0)) {
>- cond_resched();
>- latency_ration = LATENCY_LIMIT;
>- }
>- offset++;
>- }
> spin_lock(&si->lock);
>
> no_page:
>--
>2.23.0

--
Wei Yang
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