[PATCH] bio dirtying infrastructure

In two cases (AIO-for-direct-IO and some CDROM DMA stuff which Jens
did), we need to run set_page_dirty() in interrupt context.  After DMA
hardware has altered userspace pages for direct-IO reads.

But mapping->page_lock, mapping->private_lock and inode_lock are not
irq-safe.  And really, we don't want to convert those locks just for this
problem.

So what we do is to dirty the pages *before* starting I/O.  Then, in
interrupt completion context, check to see that they are still dirty.
If so then there is nothing to do.  But if the pages were cleaned while
the direct-IO is in progress we need to redirty them.  The code uses
schedule_work() for that.

Of course, we could use schedule_work() for all BIOs and pages.  The
speculative dirty-before-starting-IO is just an optimisation.  It's
quite unlikely that any of the pages will be cleaned during the direct
IO.


This code is basically untestable under normal circumstances, because the
relevant pages are pinned via get_user_pages().  This makes
is_page_cache_freeable() evaluate false and the VM doesn't try to write them
out anyway.  But if the pages happen to be MAP_SHARED file pages, pdflush
could clean them.  The way I tested this code was to disable the call to
bio_set_pages_dirty() in fs/direct-io.c.

fs/bio.c

@@ -24,6 +24,7 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/mempool.h>
+#include <linux/workqueue.h>
 
 #define BIO_POOL_SIZE 256
 
@@ -566,7 +567,112 @@ void bio_unmap_user(struct bio *bio, int write_to_vm)
 	}
 
 	bio_put(bio);
- }
+}
+
+/*
+ * bio_set_pages_dirty() and bio_check_pages_dirty() are support functions
+ * for performing direct-IO in BIOs.
+ *
+ * The problem is that we cannot run set_page_dirty() from interrupt context
+ * because the required locks are not interrupt-safe.  So what we can do is to
+ * mark the pages dirty _before_ performing IO.  And in interrupt context,
+ * check that the pages are still dirty.   If so, fine.  If not, redirty them
+ * in process context.
+ *
+ * Note that this code is very hard to test under normal circumstances because
+ * direct-io pins the pages with get_user_pages().  This makes
+ * is_page_cache_freeable return false, and the VM will not clean the pages.
+ * But other code (eg, pdflush) could clean the pages if they are mapped
+ * pagecache.
+ *
+ * Simply disabling the call to bio_set_pages_dirty() is a good way to test the
+ * deferred bio dirtying paths.
+ */
+
+/*
+ * bio_set_pages_dirty() will mark all the bio's pages as dirty.
+ */
+void bio_set_pages_dirty(struct bio *bio)
+{
+	struct bio_vec *bvec = bio->bi_io_vec;
+	int i;
+
+	for (i = 0; i < bio->bi_vcnt; i++) {
+		struct page *page = bvec[i].bv_page;
+
+		if (page)
+			set_page_dirty(bvec[i].bv_page);
+	}
+}
+
+/*
+ * bio_check_pages_dirty() will check that all the BIO's pages are still dirty.
+ * If they are, then fine.  If, however, some pages are clean then they must
+ * have been written out during the direct-IO read.  So we take another ref on
+ * the BIO and the offending pages and re-dirty the pages in process context.
+ *
+ * It is expected that bio_check_pages_dirty() will wholly own the BIO from
+ * here on.  It will run one page_cache_release() against each page and will
+ * run one bio_put() against the BIO.
+ */
+
+static void bio_dirty_fn(void *data);
+
+static DECLARE_WORK(bio_dirty_work, bio_dirty_fn, NULL);
+static spinlock_t bio_dirty_lock = SPIN_LOCK_UNLOCKED;
+static struct bio *bio_dirty_list = NULL;
+
+/*
+ * This runs in process context
+ */
+static void bio_dirty_fn(void *data)
+{
+	unsigned long flags;
+	struct bio *bio;
+
+	spin_lock_irqsave(&bio_dirty_lock, flags);
+	bio = bio_dirty_list;
+	bio_dirty_list = NULL;
+	spin_unlock_irqrestore(&bio_dirty_lock, flags);
+
+	while (bio) {
+		struct bio *next = bio->bi_private;
+
+		bio_set_pages_dirty(bio);
+		bio_put(bio);
+		bio = next;
+	}
+}
+
+void bio_check_pages_dirty(struct bio *bio)
+{
+	struct bio_vec *bvec = bio->bi_io_vec;
+	int nr_clean_pages = 0;
+	int i;
+
+	for (i = 0; i < bio->bi_vcnt; i++) {
+		struct page *page = bvec[i].bv_page;
+
+		if (PageDirty(page)) {
+			page_cache_release(page);
+			bvec[i].bv_page = NULL;
+		} else {
+			nr_clean_pages++;
+		}
+	}
+
+	if (nr_clean_pages) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&bio_dirty_lock, flags);
+		bio->bi_private = bio_dirty_list;
+		bio_dirty_list = bio;
+		spin_unlock_irqrestore(&bio_dirty_lock, flags);
+		schedule_work(&bio_dirty_work);
+	} else {
+		bio_put(bio);
+	}
+}
 
 /**
  * bio_endio - end I/O on a bio

include/linux/bio.h

@@ -219,6 +219,8 @@ extern int bio_get_nr_vecs(struct block_device *);
 extern struct bio *bio_map_user(struct block_device *, unsigned long,
 				unsigned int, int);
 extern void bio_unmap_user(struct bio *, int);
+extern void bio_set_pages_dirty(struct bio *bio);
+extern void bio_check_pages_dirty(struct bio *bio);
 
 #ifdef CONFIG_HIGHMEM
 /*