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

Johannes Weiner
2010-07-21T13:04:48Z

On Wed, Jul 21, 2010 at 12:52:50PM +0100, Mel Gorman wrote:
> On Wed, Jul 21, 2010 at 12:02:18AM +0200, Johannes Weiner wrote:
> > 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 :-)
> >
>
> Anon page writeback can enter pageout. See
>
> static inline bool reclaim_can_writeback(struct scan_control *sc,
> struct page *page)
> {
> return !page_is_file_cache(page) || current_is_kswapd();
> }
>
> So the logic still applies.

Yeah, I noticed it only after looking at it again this morning. My
bad, it got a bit late when I wrote it.

> > @@ -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));
> >
>
> We lost the __GFP_FS check and it's vaguely possible kswapd could call the
> allocator with GFP_NOFS. While you check it before wait_on_page_writeback it
> needs to be checked before calling pageout(). I toyed around with
> creating a may_pageout that took everything into account but I couldn't
> convince myself there was no holes or serious change in functionality.

Yeah, I checked balance_pgdat(), saw GFP_KERNEL and went for it. But
it's probably better to keep such dependencies out.

> Ok, is this closer to what you had in mind?

IMHO this is (almost) ready to get merged, so I am including the
nitpicking comments :-)

> ==== CUT HERE ====
> [PATCH] vmscan: Do not writeback filesystem pages in direct reclaim
>
> When memory is under enough pressure, a process may enter direct
> reclaim to free pages in the same manner kswapd does. If a dirty page is
> encountered during the scan, this page is written to backing storage using
> mapping->writepage. This can result in very deep call stacks, particularly
> if the target storage or filesystem are complex. It has already been observed
> on XFS that the stack overflows but the problem is not XFS-specific.
>
> This patch prevents direct reclaim writing back filesystem pages by checking
> if current is kswapd or the page is anonymous before writing back. If the
> dirty pages cannot be written back, they are placed back on the LRU lists
> for either background writing by the BDI threads or kswapd. If in direct
> lumpy reclaim and dirty pages are encountered, the process will stall for
> the background flusher before trying to reclaim the pages again.
>
> As the call-chain for writing anonymous pages is not expected to be deep
> and they are not cleaned by flusher threads, anonymous pages are still
> written back in direct reclaim.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: Rik van Riel <riel@redhat.com>
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6587155..e3a5816 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c

[...]

Does factoring pageout() still make sense in this patch? It does not
introduce a second callsite.

> @@ -639,18 +645,25 @@ 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

That's placed a bit randomly now that shrink_page_list() doesn't use
it anymore. I moved it just above shrink_inactive_list() but maybe it
would be better at the file's head?

> /*
> * 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)
> + enum pageout_io sync_writeback,
> + unsigned long *nr_still_dirty)
> {
> - LIST_HEAD(ret_pages);
> LIST_HEAD(free_pages);
> - int pgactivate = 0;
> + LIST_HEAD(putback_pages);
> + LIST_HEAD(dirty_pages);
> + int pgactivate;
> + unsigned long nr_dirty = 0;
> unsigned long nr_reclaimed = 0;
>
> + pgactivate = 0;

Spurious change?

> cond_resched();
>
> while (!list_empty(page_list)) {
> @@ -741,7 +754,18 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> }
> }
>
> - if (PageDirty(page)) {
> + if (PageDirty(page)) {

Ha!

> + /*
> + * Only kswapd can writeback filesystem pages to
> + * avoid risk of stack overflow
> + */
> + if (page_is_file_cache(page) && !current_is_kswapd()) {
> + list_add(&page->lru, &dirty_pages);
> + unlock_page(page);
> + nr_dirty++;
> + goto keep_dirty;
> + }

I don't understand why you keep the extra dirty list. Couldn't this
just be `goto keep_locked'?

> if (references == PAGEREF_RECLAIM_CLEAN)
> goto keep_locked;
> if (!may_enter_fs)
> @@ -852,13 +876,19 @@ 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));
> }
>
> free_page_list(&free_pages);
>
> - list_splice(&ret_pages, page_list);
> + if (nr_dirty) {
> + *nr_still_dirty = nr_dirty;

You either have to set *nr_still_dirty unconditionally or
(re)initialize the variable in shrink_inactive_list().

> + list_splice(&dirty_pages, page_list);
> + }
> + list_splice(&putback_pages, page_list);

When we retry those pages, the dirty ones come last on the list. Was
this maybe the intention behind collecting dirties separately?

> @@ -1245,6 +1275,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> 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);
> @@ -1293,26 +1324,34 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
>
> 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
> + * If specific pages are needed such as with direct reclaiming
> + * for contiguous pages or for memory containers 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
> + * for IO to complete. This will stall callers that require
> + * specific pages but it 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 (sc->may_writepage && !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(laptop_mode ? 0 : nr_dirty);

Yup, minding laptop_mode (together with may_writepage). Agreed.

> + congestion_wait(BLK_RW_ASYNC, HZ/10);
>
> - nr_reclaimed += shrink_page_list(&page_list, sc, PAGEOUT_IO_SYNC);
> + /*
> + * 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_dirty);
> + }
> }
>
> local_irq_disable();

Thanks,
Hannes