mm: move pagevec stripping to save unlock-relock

In shrink_active_list() after the deactivation loop, we strip buffer heads from the potentially remaining pages in the pagevec.

Currently, this drops the zone's lru lock for stripping, only to reacquire it again afterwards to update statistics.

It is not necessary to strip the pages before updating the stats, so move the whole thing out of the protected region and save the extra locking.

Signed-off-by Johannes Weiner <[email protected]>

---
 mm/vmscan.c |    7 ++-----

 1 file changed, 2 insertions(+), 5 deletions(-)

mm/vmscan.c +2 -6

@@ -1298,14 +1298,11 @@ static void shrink_active_list(unsigned
        }
        __mod_zone_page_state(zone, NR_LRU_BASE + lru, pgmoved);
        pgdeactivate += pgmoved;
-       if (buffer_heads_over_limit) {
-               spin_unlock_irq(&zone->lru_lock);
-               pagevec_strip(&pvec);
-               spin_lock_irq(&zone->lru_lock);
-       }
        __count_zone_vm_events(PGREFILL, zone, pgscanned);
        __count_vm_events(PGDEACTIVATE, pgdeactivate);
        spin_unlock_irq(&zone->lru_lock);
+       if (buffer_heads_over_limit)
+               pagevec_strip(&pvec);
        if (vm_swap_full())
                pagevec_swap_free(&pvec);
 
 
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The pagevec_swap_free() at the end of shrink_active_list() was introduced in 68a22394 "vmscan: free swap space on swap-in/activation" when shrink_active_list() was still rotating referenced active pages.

In 7e9cd48 "vmscan: fix pagecache reclaim referenced bit check" this was changed, the rotating removed but the pagevec_swap_free() after the rotation loop was forgotten, applying now to the pagevec of the deactivation loop instead.

Now swap space is freed for deactivated pages. And only for those that happen to be on the pagevec after the deactivation loop.

Complete 7e9cd48 and remove the rest of the swap freeing.

Signed-off-by Johannes Weiner <[email protected]>

Cc Rik van Riel <[email protected]>

---
 mm/vmscan.c |    3 ---
 1 file changed, 3 deletions(-)

Quoted 10 lines

The pagevec_swap_free() at the end of shrink_active_list() was introduced in 68a22394 "vmscan: free swap space on swap-in/activation" when shrink_active_list() was still rotating referenced active pages.

In 7e9cd48 "vmscan: fix pagecache reclaim referenced bit check" this was changed, the rotating removed but the pagevec_swap_free() after the rotation loop was forgotten, applying now to the pagevec of the deactivation loop instead.

Quoted 7 lines

Now swap space is freed for deactivated pages. And only for those that happen to be on the pagevec after the deactivation loop.

Complete 7e9cd48 and remove the rest of the swap freeing.

Signed-off-by Johannes Weiner <[email protected]>

Cc Rik van Riel <[email protected]>

Nice observation. I was going to object that the original code was indifferent to whether it was freeing swap from active or inactive, they all got lumped into the same pvec. But that was just an oversight in the original code: you're right that if it was our intention to free swap from inactive pages here (when vm_swap_full), then we'd be freeing it from them in the loop above (where the buffer_heads_over_limit pagevec_strip is done).

Once upon a time (early 2007), testing an earlier incarnation of that code, I did find almost nothing being freed by that pagevec_swap_free
anyway: other vm_swap_full frees were being effective, effective enough to render this one rather pointless, even when it was operating as intended. But I never got around to checking on that in 2008's splitLRU patches, and a lot changed in between: I may be misleading.

If Rik agrees (I think these do need his Ack), note that there are no other users of pagevec_swap_free, so you'd do well to remove it from mm/swap.c and include/linux/pagevec.h - I can well imagine us wanting to bring it back some time, but can easily look it up when and if we do need it again in the future.

Subject: [PATCH] remove pagevec_swap_free()

pagevec_swap_free() is unused.
then it can be removed.

Signed-off-by KOSAKI Motohiro <[email protected]>

The pagevec_swap_free() at the end of shrink_active_list() was introduced in 68a22394 "vmscan: free swap space on swap-in/activation" when shrink_active_list() was still rotating referenced active pages.

In 7e9cd48 "vmscan: fix pagecache reclaim referenced bit check" this was changed, the rotating removed but the pagevec_swap_free() after the rotation loop was forgotten, applying now to the pagevec of the deactivation loop instead.

Now swap space is freed for deactivated pages. And only for those that happen to be on the pagevec after the deactivation loop.

Complete 7e9cd48 and remove the rest of the swap freeing.

Signed-off-by Johannes Weiner <[email protected]>

Cc Rik van Riel <[email protected]>