(Requires the direct-to-BIO-for-swap patch)
One weakness which was introduced when the buffer LRU went away was
that GFP_NOFS allocations became equivalent to GFP_NOIO. Because all
writeback goes via writepage/writepages, which requires entry into the
filesystem.
However now that swapout no longer calls bmap(), we can honour
GFP_NOFS's intent for swapcache pages. So if the allocation request
specifies __GFP_IO and !__GFP_FS, we can wait on swapcache pages and we
can perform swapcache writeout.
This should strengthen the VM somewhat.
--- 2.5.22/mm/vmscan.c~GFP_IO-swap Sun Jun 16 23:12:53 2002
+++ 2.5.22-akpm/mm/vmscan.c Sun Jun 16 23:12:53 2002
@@ -391,7 +391,8 @@ shrink_cache(int nr_pages, zone_t *class
spin_lock(&pagemap_lru_lock);
while (--max_scan >= 0 &&
(entry = inactive_list.prev) != &inactive_list) {
- struct page * page;
+ struct page *page;
+ int may_enter_fs;
if (need_resched()) {
spin_unlock(&pagemap_lru_lock);
@@ -426,10 +427,17 @@ shrink_cache(int nr_pages, zone_t *class
goto page_mapped;
/*
+ * swap activity never enters the filesystem and is safe
+ * for GFP_NOFS allocations.
+ */
+ may_enter_fs = (gfp_mask & __GFP_FS) ||
+ (PageSwapCache(page) && (gfp_mask & __GFP_IO));
+
+ /*
* IO in progress? Leave it at the back of the list.
*/
if (unlikely(PageWriteback(page))) {
- if (gfp_mask & __GFP_FS) {
+ if (may_enter_fs) {
page_cache_get(page);
spin_unlock(&pagemap_lru_lock);
wait_on_page_writeback(page);
@@ -450,7 +458,7 @@ shrink_cache(int nr_pages, zone_t *class
mapping = page->mapping;
if (PageDirty(page) && is_page_cache_freeable(page) &&
- page->mapping && (gfp_mask & __GFP_FS)) {
+ page->mapping && may_enter_fs) {
/*
* It is not critical here to write it only if
* the page is unmapped beause any direct writer
@@ -479,6 +487,15 @@ shrink_cache(int nr_pages, zone_t *class
* If the page has buffers, try to free the buffer mappings
* associated with this page. If we succeed we try to free
* the page as well.
+ *
+ * We do this even if the page is PageDirty().
+ * try_to_release_page() does not perform I/O, but it is
+ * possible for a page to have PageDirty set, but it is actually
+ * clean (all its buffers are clean). This happens if the
+ * buffers were written out directly, with submit_bh(). ext3
+ * will do this, as well as the blockdev mapping.
+ * try_to_release_page() will discover that cleanness and will
+ * drop the buffers and mark the page clean - it can be freed.
*/
if (PagePrivate(page)) {
spin_unlock(&pagemap_lru_lock);
-
-
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