Re: [PATCH 4/8] vmscan: Do not writeback filesystem pages in direct reclaim

Johannes Weiner
2010-07-21T11:37:06Z

On Wed, Jul 21, 2010 at 12:02:18AM +0200, Johannes Weiner wrote:
> On Tue, Jul 20, 2010 at 02:45:56PM +0100, Mel Gorman wrote:
> > On Tue, Jul 20, 2010 at 12:14:20AM +0200, Johannes Weiner wrote:
> > > I think it would turn out more natural to just return dirty pages on
> > > page_list and have the whole looping logic in shrink_inactive_list().
> > >
> > > Mixing dirty pages with other 'please try again' pages is probably not
> > > so bad anyway, it means we could retry all temporary unavailable pages
> > > instead of twiddling thumbs over that particular bunch of pages until
> > > the flushers catch up.
> > >
> > > What do you think?
> > >
[...]
> > The reason why I did it this way was because of lumpy reclaim and memcg
> > requiring specific pages. I considered lumpy reclaim to be the more common
> > case. In that case, it's removing potentially a large number of pages from
> > the LRU that are contiguous. If some of those are dirty and it selects more
> > contiguous ranges for reclaim, I'd worry that lumpy reclaim would trash the
> > system even worse than it currently does when the system is under load. Hence,
> > this wait and retry loop is done instead of returning and isolating more pages.
>
> I think here we missed each other. I don't want the loop to be _that_
> much more in the outer scope that isolation is repeated as well. What
> I had in mind is the attached patch. It is not tested and hacked up
> rather quickly due to time constraints, sorry, but you should get the
> idea. I hope I did not miss anything fundamental.
>
> Note that since only kswapd enters pageout() anymore, everything
> depending on PAGEOUT_IO_SYNC in there is moot, since there are no sync
> cycles for kswapd. Just to mitigate the WTF-count on the patch :-)

Aaaaand direct reclaimers for swap, of course. Selfslap. Here is the
patch again, sans the first hunk (and the type of @dirty_seen fixed):

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -643,12 +643,14 @@ static noinline_for_stack void free_page
* shrink_page_list() returns the number of reclaimed pages
*/
static unsigned long shrink_page_list(struct list_head *page_list,
- struct scan_control *sc,
- enum pageout_io sync_writeback)
+ struct scan_control *sc,
+ enum pageout_io sync_writeback,
+ unsigned long *dirty_seen)
{
LIST_HEAD(ret_pages);
LIST_HEAD(free_pages);
int pgactivate = 0;
+ unsigned long nr_dirty = 0;
unsigned long nr_reclaimed = 0;

cond_resched();
@@ -657,7 +659,7 @@ static unsigned long shrink_page_list(st
enum page_references references;
struct address_space *mapping;
struct page *page;
- int may_enter_fs;
+ int may_pageout;

cond_resched();

@@ -681,10 +683,15 @@ static unsigned long shrink_page_list(st
if (page_mapped(page) || PageSwapCache(page))
sc->nr_scanned++;

- may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
+ /*
+ * To prevent stack overflows, only kswapd can enter
+ * the filesystem. Swap IO is always fine (for now).
+ */
+ may_pageout = current_is_kswapd() ||
(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));

if (PageWriteback(page)) {
+ int may_wait;
/*
* Synchronous reclaim is performed in two passes,
* first an asynchronous pass over the list to
@@ -693,7 +700,8 @@ static unsigned long shrink_page_list(st
* for any page for which writeback has already
* started.
*/
- if (sync_writeback == PAGEOUT_IO_SYNC && may_enter_fs)
+ may_wait = (sc->gfp_mask & __GFP_FS) || may_pageout;
+ if (sync_writeback == PAGEOUT_IO_SYNC && may_wait)
wait_on_page_writeback(page);
else
goto keep_locked;
@@ -719,7 +727,7 @@ static unsigned long shrink_page_list(st
goto keep_locked;
if (!add_to_swap(page))
goto activate_locked;
- may_enter_fs = 1;
+ may_pageout = 1;
}

mapping = page_mapping(page);
@@ -742,9 +750,11 @@ static unsigned long shrink_page_list(st
}

if (PageDirty(page)) {
+ nr_dirty++;
+
if (references == PAGEREF_RECLAIM_CLEAN)
goto keep_locked;
- if (!may_enter_fs)
+ if (!may_pageout)
goto keep_locked;
if (!sc->may_writepage)
goto keep_locked;
@@ -860,6 +870,7 @@ keep:

list_splice(&ret_pages, page_list);
count_vm_events(PGACTIVATE, pgactivate);
+ *dirty_seen = nr_dirty;
return nr_reclaimed;
}

@@ -1232,6 +1243,9 @@ static noinline_for_stack void update_is
reclaim_stat->recent_scanned[1] += *nr_file;
}

+/* Direct lumpy reclaim waits up to 5 seconds for background cleaning */
+#define MAX_SWAP_CLEAN_WAIT 50
+
/*
* shrink_inactive_list() is a helper for shrink_zone(). It returns the number
* of reclaimed pages
@@ -1247,6 +1261,7 @@ shrink_inactive_list(unsigned long nr_to
unsigned long nr_active;
unsigned long nr_anon;
unsigned long nr_file;
+ unsigned long nr_dirty;

while (unlikely(too_many_isolated(zone, file, sc))) {
congestion_wait(BLK_RW_ASYNC, HZ/10);
@@ -1295,26 +1310,32 @@ shrink_inactive_list(unsigned long nr_to

spin_unlock_irq(&zone->lru_lock);

- nr_reclaimed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);
-
+ nr_reclaimed = shrink_page_list(&page_list, sc,
+ PAGEOUT_IO_ASYNC,
+ &nr_dirty);
/*
* If we are direct reclaiming for contiguous pages and we do
* not reclaim everything in the list, try again and wait
* for IO to complete. This will stall high-order allocations
* but that should be acceptable to the caller
*/
- if (nr_reclaimed < nr_taken && !current_is_kswapd() &&
- sc->lumpy_reclaim_mode) {
- congestion_wait(BLK_RW_ASYNC, HZ/10);
+ if (!current_is_kswapd() && sc->lumpy_reclaim_mode || sc->mem_cgroup) {
+ int dirty_retry = MAX_SWAP_CLEAN_WAIT;

- /*
- * The attempt at page out may have made some
- * of the pages active, mark them inactive again.
- */
- nr_active = clear_active_flags(&page_list, NULL);
- count_vm_events(PGDEACTIVATE, nr_active);
+ while (nr_reclaimed < nr_taken && nr_dirty && dirty_retry--) {
+ wakeup_flusher_threads(nr_dirty);
+ congestion_wait(BLK_RW_ASYNC, HZ/10);
+ /*
+ * The attempt at page out may have made some
+ * of the pages active, mark them inactive again.
+ */
+ nr_active = clear_active_flags(&page_list, NULL);
+ count_vm_events(PGDEACTIVATE, nr_active);

- nr_reclaimed += shrink_page_list(&page_list, sc, PAGEOUT_IO_SYNC);
+ nr_reclaimed += shrink_page_list(&page_list, sc,
+ PAGEOUT_IO_SYNC,
+ &nr_dirty);
+ }
}

local_irq_disable();