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

Johannes Weiner
2010-07-20T22:02:31Z

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:
> > > @@ -639,6 +694,9 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
> > > pagevec_free(&freed_pvec);
> > > }
> > >
> > > +/* Direct lumpy reclaim waits up to 5 seconds for background cleaning */
> > > +#define MAX_SWAP_CLEAN_WAIT 50
> > > +
> > > /*
> > > * shrink_page_list() returns the number of reclaimed pages
> > > */
> > > @@ -646,13 +704,19 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > > struct scan_control *sc,
> > > enum pageout_io sync_writeback)
> > > {
> > > - LIST_HEAD(ret_pages);
> > > LIST_HEAD(free_pages);
> > > - int pgactivate = 0;
> > > + LIST_HEAD(putback_pages);
> > > + LIST_HEAD(dirty_pages);
> > > + int pgactivate;
> > > + int dirty_isolated = 0;
> > > + unsigned long nr_dirty;
> > > unsigned long nr_reclaimed = 0;
> > >
> > > + pgactivate = 0;
> > > cond_resched();
> > >
> > > +restart_dirty:
> > > + nr_dirty = 0;
> > > while (!list_empty(page_list)) {
> > > enum page_references references;
> > > struct address_space *mapping;
> > > @@ -741,7 +805,19 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > > }
> > > }
> > >
> > > - if (PageDirty(page)) {
> > > + if (PageDirty(page)) {
> > > + /*
> > > + * If the caller cannot writeback pages, dirty pages
> > > + * are put on a separate list for cleaning by either
> > > + * a flusher thread or kswapd
> > > + */
> > > + if (!reclaim_can_writeback(sc, page)) {
> > > + list_add(&page->lru, &dirty_pages);
> > > + unlock_page(page);
> > > + nr_dirty++;
> > > + goto keep_dirty;
> > > + }
> > > +
> > > if (references == PAGEREF_RECLAIM_CLEAN)
> > > goto keep_locked;
> > > if (!may_enter_fs)
> > > @@ -852,13 +928,39 @@ activate_locked:
> > > keep_locked:
> > > unlock_page(page);
> > > keep:
> > > - list_add(&page->lru, &ret_pages);
> > > + list_add(&page->lru, &putback_pages);
> > > +keep_dirty:
> > > VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
> > > }
> > >
> > > + if (dirty_isolated < MAX_SWAP_CLEAN_WAIT && !list_empty(&dirty_pages)) {
> > > + /*
> > > + * Wakeup a flusher thread to clean at least as many dirty
> > > + * pages as encountered by direct reclaim. Wait on congestion
> > > + * to throttle processes cleaning dirty pages
> > > + */
> > > + wakeup_flusher_threads(nr_dirty);
> > > + congestion_wait(BLK_RW_ASYNC, HZ/10);
> > > +
> > > + /*
> > > + * As lumpy reclaim and memcg targets specific pages, wait on
> > > + * them to be cleaned and try reclaim again.
> > > + */
> > > + if (sync_writeback == PAGEOUT_IO_SYNC ||
> > > + sc->mem_cgroup != NULL) {
> > > + dirty_isolated++;
> > > + list_splice(&dirty_pages, page_list);
> > > + INIT_LIST_HEAD(&dirty_pages);
> > > + goto restart_dirty;
> > > + }
> > > + }
> >
> > 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?
> >
>
> It's worth considering! It won't be very tidy but it's workable. The reason
> it is not tidy is that dirty pages and pages that couldn't be paged will be
> on the same list so they whole lot will need to be recycled. We'd record in
> scan_control though that there were pages that need to be retried and loop
> based on that value. That is managable though.

Recycling all of them is what I had in mind, yeah. But...

> 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 :-)

Hannes

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -386,21 +386,17 @@ static pageout_t pageout(struct page *pa
ClearPageReclaim(page);
return PAGE_ACTIVATE;
}
-
- /*
- * Wait on writeback if requested to. This happens when
- * direct reclaiming a large contiguous area and the
- * first attempt to free a range of pages fails.
- */
- if (PageWriteback(page) && sync_writeback == PAGEOUT_IO_SYNC)
- wait_on_page_writeback(page);
-
if (!PageWriteback(page)) {
/* synchronous write or broken a_ops? */
ClearPageReclaim(page);
}
trace_mm_vmscan_writepage(page,
page_is_file_cache(page),
+ /*
+ * Humm. Only kswapd comes here and for
+ * kswapd there never is a PAGEOUT_IO_SYNC
+ * cycle...
+ */
sync_writeback == PAGEOUT_IO_SYNC);
inc_zone_page_state(page, NR_VMSCAN_WRITE);
return PAGE_SUCCESS;
@@ -643,12 +639,14 @@ 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,
+ int *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 +655,7 @@ 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 +679,15 @@ 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 +696,8 @@ 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 +723,7 @@ 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 +746,11 @@ 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 +866,7 @@ keep:

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

@@ -1232,6 +1239,9 @@ 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 +1257,7 @@ 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 +1306,34 @@ 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.
+ *
+ * Humm. Still needed?
+ */
+ 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();