[PATCH] better drop-behind in generic_file_write

[Rik van Riel <riel@conectiva.com.br>]

Hi Linus, Daniel,

the (trivial) patch below fixes the drop-behind call in
generic_file_write to *only* do drop-behind if we've
written "past the end" of the page.

This way we have a better chance of still having partially
written pages in memory when we write to them again (eg. for
TUX logfiles). Not only will this speed up non page-aligned
writes on loaded systems, it'll also give slower writes a
small advantage over fast writes (which will skip page boundaries
more often).

Linus, Alan, could you please apply this patch in your next
2.4.0-test kernel ?

regards,

Rik
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--- linux-2.4.0-test12-pre3/mm/filemap.c.orig	2000/12/21 18:20:17
+++ linux/mm/filemap.c	2000/12/21 21:31:39
@@ -2436,7 +2436,7 @@
 	}
 
 	while (count) {
-		unsigned long bytes, index, offset;
+		unsigned long bytes, index, offset, partial = 0;
 		char *kaddr;
 
 		/*
@@ -2446,8 +2446,10 @@
 		offset = (pos & (PAGE_CACHE_SIZE -1)); /* Within page */
 		index = pos >> PAGE_CACHE_SHIFT;
 		bytes = PAGE_CACHE_SIZE - offset;
-		if (bytes > count)
+		if (bytes > count) {
 			bytes = count;
+			partial = 1;
+		}
 
 		status = -ENOMEM;	/* we'll assign it later anyway */
 		page = __grab_cache_page(mapping, index, &cached_page);
@@ -2478,9 +2480,17 @@
 			buf += status;
 		}
 unlock:
-		/* Mark it unlocked again and drop the page.. */
+		/*
+		 * Mark it unlocked again and release the page.
+		 * In order to prevent large (fast) file writes
+		 * from causing too much memory pressure we move
+		 * completely written pages to the inactive list.
+		 * We do, however, try to keep the pages that may
+		 * still be written to (ie. partially written pages).
+		 */
 		UnlockPage(page);
-		deactivate_page(page);
+		if (!partial)
+			deactivate_page(page);
 		page_cache_release(page);
 
 		if (status < 0)


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