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Subject[PATCH] vfs: reduce stack usage in write_cache_pages()
Date
Hi Al, David,

vfs function write_cache_pages() appears on the xfs
writeout path. It's *the* path which causes 4k stack
overflows on i386 with xfs.

This function uses ~100 bytes of stack on 32-bit i386.

This patch transforms it a little so that it uses
a bit less stack - minus 8 bytes to be precise.

This isn't much, but it helps not only xfs,
but all filesystems.

Only compile tested.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
--
vda
diff -urpN linux-2.6-xfs1/mm/page-writeback.c linux-2.6-xfs1.stk3/mm/page-writeback.c
--- linux-2.6-xfs1/mm/page-writeback.c 2008-03-30 03:27:55.000000000 +0200
+++ linux-2.6-xfs1.stk3/mm/page-writeback.c 2008-04-27 04:14:39.000000000 +0200
@@ -798,17 +798,14 @@ int write_cache_pages(struct address_spa
struct writeback_control *wbc, writepage_t writepage,
void *data)
{
- struct backing_dev_info *bdi = mapping->backing_dev_info;
int ret = 0;
int done = 0;
struct pagevec pvec;
- int nr_pages;
pgoff_t index;
pgoff_t end; /* Inclusive */
int scanned = 0;
- int range_whole = 0;

- if (wbc->nonblocking && bdi_write_congested(bdi)) {
+ if (wbc->nonblocking && bdi_write_congested(mapping->backing_dev_info)) {
wbc->encountered_congestion = 1;
return 0;
}
@@ -820,20 +817,30 @@ int write_cache_pages(struct address_spa
} else {
index = wbc->range_start >> PAGE_CACHE_SHIFT;
end = wbc->range_end >> PAGE_CACHE_SHIFT;
- if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
- range_whole = 1;
scanned = 1;
}
+
+ /* Minimizing stack use:
+ * "nr_pages" hopefully won't require a stack slot -
+ * we reuse "scanned" to keep its value.
+ */
+
retry:
- while (!done && (index <= end) &&
- (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
- PAGECACHE_TAG_DIRTY,
- min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
+ while (!done && (index <= end)) {
unsigned i;
+ int nr_pages;

- scanned = 1;
- for (i = 0; i < nr_pages; i++) {
- struct page *page = pvec.pages[i];
+ nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
+ PAGECACHE_TAG_DIRTY,
+ min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
+ if (!nr_pages)
+ break;
+ scanned = nr_pages;
+ /* "scanned" counts down to 1, and later acts as nonzero flag */
+
+ i = (unsigned)-1;
+ while (1) { /* for i in [0..nr_pages-1] */
+ struct page *page = pvec.pages[++i];

/*
* At this point we hold neither mapping->tree_lock nor
@@ -872,10 +879,13 @@ retry:
}
if (ret || (--(wbc->nr_to_write) <= 0))
done = 1;
- if (wbc->nonblocking && bdi_write_congested(bdi)) {
+ if (wbc->nonblocking && bdi_write_congested(mapping->backing_dev_info)) {
wbc->encountered_congestion = 1;
done = 1;
}
+ if (scanned == 1)
+ break;
+ scanned--;
}
pagevec_release(&pvec);
cond_resched();
@@ -889,7 +899,8 @@ retry:
index = 0;
goto retry;
}
- if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
+ if (wbc->range_cyclic || (wbc->range_start == 0 && wbc->range_end == LLONG_MAX /* whole range */
+ && wbc->nr_to_write > 0))
mapping->writeback_index = index;
return ret;
}
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