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Subject[patch 04/22] fix deadlock in throttle_vm_writeout
From: Miklos Szeredi <mszeredi@suse.cz>

This deadlock is similar to the one in balance_dirty_pages, but
instead of waiting in balance_dirty_pages after submitting a write
request, it happens during a memory allocation for filesystem B before
submitting a write request.

It is easy to reproduce on a machine with not too much memory.
E.g. try this on 2.6.21-rc1 UML with 32MB (works on physical hw as
well):

dd if=/dev/zero of=/tmp/tmp.img bs=1048576 count=40
mke2fs -j -F /tmp/tmp.img
mkdir /tmp/img
mount -oloop /tmp/tmp.img /tmp/img
bash-shared-mapping /tmp/img/foo 30000000
The deadlock doesn't happen immediately, sometimes only after a few
minutes.

Simplified stack trace for bash-shared-mapping after the deadlock:

io_schedule_timeout
congestion_wait
balance_dirty_pages
balance_dirty_pages_ratelimited_nr
generic_file_buffered_write
__generic_file_aio_write_nolock
generic_file_aio_write
ext3_file_write
do_sync_write
vfs_write
sys_pwrite64

and for [loop0]:

io_schedule_timeout
congestion_wait
throttle_vm_writeout
shrink_zone
shrink_zones
try_to_free_pages
__alloc_pages
find_or_create_page
do_lo_send_aops
lo_send
do_bio_filebacked
loop_thread

The requirement for the deadlock is that

nr_writeback > dirty_thresh * 1.1 + margin
Again margin seems to be in the 100 page range.

The task of throttle_vm_writeout is to limit the rate at which
under-writeback pages are created due to swapping. There's no other
way direct reclaim can increase the nr_writeback + nr_file_dirty.

So when there are few or no under-swap pages, it is safe for this
function to return. This ensures, that there's progress with writing
back dirty pages.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---
Index: linux/include/linux/swap.h
===================================================================
--- linux.orig/include/linux/swap.h 2007-02-27 14:40:55.000000000 +0100
+++ linux/include/linux/swap.h 2007-02-27 14:41:08.000000000 +0100
@@ -279,10 +279,14 @@ static inline void disable_swap_token(vo
put_swap_token(swap_token_mm);
}

+#define nr_swap_writeback \
+ atomic_long_read(&swapper_space.backing_dev_info->nr_writeback)
+
#else /* CONFIG_SWAP */

#define total_swap_pages 0
#define total_swapcache_pages 0UL
+#define nr_swap_writeback 0UL

#define si_swapinfo(val) \
do { (val)->freeswap = (val)->totalswap = 0; } while (0)
Index: linux/mm/page-writeback.c
===================================================================
--- linux.orig/mm/page-writeback.c 2007-02-27 14:41:07.000000000 +0100
+++ linux/mm/page-writeback.c 2007-02-27 14:41:08.000000000 +0100
@@ -33,6 +33,7 @@
#include <linux/syscalls.h>
#include <linux/buffer_head.h>
#include <linux/pagevec.h>
+#include <linux/swap.h>

/*
* The maximum number of pages to writeout in a single bdflush/kupdate
@@ -303,6 +304,21 @@ void throttle_vm_writeout(void)
long dirty_thresh;

for ( ; ; ) {
+ /*
+ * If there's no swapping going on, don't throttle.
+ *
+ * Starting writeback against mapped pages shouldn't
+ * be a problem, as that doesn't increase the
+ * sum of dirty + writeback.
+ *
+ * Without this, a deadlock is possible (also see
+ * comment in balance_dirty_pages). This has been
+ * observed with running bash-shared-mapping on a
+ * loopback mount.
+ */
+ if (nr_swap_writeback < 16)
+ break;
+
get_dirty_limits(&background_thresh, &dirty_thresh, NULL);

/*
@@ -314,6 +330,7 @@ void throttle_vm_writeout(void)
if (global_page_state(NR_UNSTABLE_NFS) +
global_page_state(NR_WRITEBACK) <= dirty_thresh)
break;
+
congestion_wait(WRITE, HZ/10);
}
}
Index: linux/mm/page_io.c
===================================================================
--- linux.orig/mm/page_io.c 2007-02-27 14:40:55.000000000 +0100
+++ linux/mm/page_io.c 2007-02-27 14:41:08.000000000 +0100
@@ -70,6 +70,7 @@ static int end_swap_bio_write(struct bio
ClearPageReclaim(page);
}
end_page_writeback(page);
+ atomic_long_dec(&swapper_space.backing_dev_info->nr_writeback);
bio_put(bio);
return 0;
}
@@ -121,6 +122,7 @@ int swap_writepage(struct page *page, st
if (wbc->sync_mode == WB_SYNC_ALL)
rw |= (1 << BIO_RW_SYNC);
count_vm_event(PSWPOUT);
+ atomic_long_inc(&swapper_space.backing_dev_info->nr_writeback);
set_page_writeback(page);
unlock_page(page);
submit_bio(rw, bio);
--
-
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