[PATCH 23/42] Unionfs: address-space operations

[Erez Zadok]

Includes writepage, writepages, readpage, prepare_write, and commit_write.

Signed-off-by: Erez Zadok <ezk@xxxxxxxxxxxxx>
---
 fs/unionfs/mmap.c |  338 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 338 insertions(+), 0 deletions(-)
 create mode 100644 fs/unionfs/mmap.c

diff --git a/fs/unionfs/mmap.c b/fs/unionfs/mmap.c
new file mode 100644
index 0000000..4d05352
--- /dev/null
+++ b/fs/unionfs/mmap.c
@@ -0,0 +1,338 @@
+/*
+ * Copyright (c) 2003-2007 Erez Zadok
+ * Copyright (c) 2003-2006 Charles P. Wright
+ * Copyright (c) 2005-2007 Josef 'Jeff' Sipek
+ * Copyright (c) 2005-2006 Junjiro Okajima
+ * Copyright (c) 2006      Shaya Potter
+ * Copyright (c) 2005      Arun M. Krishnakumar
+ * Copyright (c) 2004-2006 David P. Quigley
+ * Copyright (c) 2003-2004 Mohammad Nayyer Zubair
+ * Copyright (c) 2003      Puja Gupta
+ * Copyright (c) 2003      Harikesavan Krishnan
+ * Copyright (c) 2003-2007 Stony Brook University
+ * Copyright (c) 2003-2007 The Research Foundation of SUNY
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include "union.h"
+
+static int unionfs_writepage(struct page *page, struct writeback_control *wbc)
+{
+	int err = -EIO;
+	struct inode *inode;
+	struct inode *lower_inode;
+	struct page *lower_page;
+	struct address_space *lower_mapping; /* lower inode mapping */
+	gfp_t mask;
+
+	BUG_ON(!PageUptodate(page));
+	inode = page->mapping->host;
+	lower_inode = unionfs_lower_inode(inode);
+	lower_mapping = lower_inode->i_mapping;
+
+	/*
+	 * find lower page (returns a locked page)
+	 *
+	 * We turn off __GFP_FS while we look for or create a new lower
+	 * page.  This prevents a recursion into the file system code, which
+	 * under memory pressure conditions could lead to a deadlock.  This
+	 * is similar to how the loop driver behaves (see loop_set_fd in
+	 * drivers/block/loop.c).  If we can't find the lower page, we
+	 * redirty our page and return "success" so that the VM will call us
+	 * again in the (hopefully near) future.
+	 */
+	mask = mapping_gfp_mask(lower_mapping) & ~(__GFP_FS);
+	lower_page = find_or_create_page(lower_mapping, page->index, mask);
+	if (!lower_page) {
+		err = 0;
+		set_page_dirty(page);
+		goto out;
+	}
+
+	/* copy page data from our upper page to the lower page */
+	copy_highpage(lower_page, page);
+	flush_dcache_page(lower_page);
+	SetPageUptodate(lower_page);
+	set_page_dirty(lower_page);
+
+	/*
+	 * Call lower writepage (expects locked page).  However, if we are
+	 * called with wbc->for_reclaim, then the VFS/VM just wants to
+	 * reclaim our page.  Therefore, we don't need to call the lower
+	 * ->writepage: just copy our data to the lower page (already done
+	 * above), then mark the lower page dirty and unlock it, and return
+	 * success.
+	 */
+	if (wbc->for_reclaim) {
+		unlock_page(lower_page);
+		goto out_release;
+	}
+
+	BUG_ON(!lower_mapping->a_ops->writepage);
+	wait_on_page_writeback(lower_page); /* prevent multiple writers */
+	clear_page_dirty_for_io(lower_page); /* emulate VFS behavior */
+	err = lower_mapping->a_ops->writepage(lower_page, wbc);
+	if (err < 0)
+		goto out_release;
+
+	/*
+	 * Lower file systems such as ramfs and tmpfs, may return
+	 * AOP_WRITEPAGE_ACTIVATE so that the VM won't try to (pointlessly)
+	 * write the page again for a while.  But those lower file systems
+	 * also set the page dirty bit back again.  Since we successfully
+	 * copied our page data to the lower page, then the VM will come
+	 * back to the lower page (directly) and try to flush it.  So we can
+	 * save the VM the hassle of coming back to our page and trying to
+	 * flush too.  Therefore, we don't re-dirty our own page, and we
+	 * never return AOP_WRITEPAGE_ACTIVATE back to the VM (we consider
+	 * this a success).
+	 *
+	 * We also unlock the lower page if the lower ->writepage returned
+	 * AOP_WRITEPAGE_ACTIVATE.  (This "anomalous" behaviour may be
+	 * addressed in future shmem/VM code.)
+	 */
+	if (err == AOP_WRITEPAGE_ACTIVATE) {
+		err = 0;
+		unlock_page(lower_page);
+	}
+
+	/* all is well */
+
+	/* lower mtimes have changed: update ours */
+	unionfs_copy_attr_times(inode);
+
+out_release:
+	/* b/c find_or_create_page increased refcnt */
+	page_cache_release(lower_page);
+out:
+	/*
+	 * We unlock our page unconditionally, because we never return
+	 * AOP_WRITEPAGE_ACTIVATE.
+	 */
+	unlock_page(page);
+	return err;
+}
+
+static int unionfs_writepages(struct address_space *mapping,
+			      struct writeback_control *wbc)
+{
+	int err = 0;
+	struct inode *lower_inode;
+	struct inode *inode;
+
+	inode = mapping->host;
+	if (ibstart(inode) < 0 && ibend(inode) < 0)
+		goto out;
+	lower_inode = unionfs_lower_inode(inode);
+	if (!lower_inode)
+		goto out;
+
+	if (!mapping_cap_writeback_dirty(lower_inode->i_mapping))
+		goto out;
+
+	err = generic_writepages(mapping, wbc);
+	if (!err)
+		unionfs_copy_attr_times(inode);
+out:
+	return err;
+}
+
+/* Readpage expects a locked page, and must unlock it */
+static int unionfs_readpage(struct file *file, struct page *page)
+{
+	int err;
+	struct file *lower_file;
+	struct inode *inode;
+	mm_segment_t old_fs;
+	char *page_data = NULL;
+	mode_t orig_mode;
+
+	unionfs_read_lock(file->f_path.dentry->d_sb);
+	err = unionfs_file_revalidate(file, false);
+	if (unlikely(err))
+		goto out;
+	unionfs_check_file(file);
+
+	if (!UNIONFS_F(file)) {
+		err = -ENOENT;
+		goto out;
+	}
+
+	lower_file = unionfs_lower_file(file);
+	/* FIXME: is this assertion right here? */
+	BUG_ON(lower_file == NULL);
+
+	inode = file->f_path.dentry->d_inode;
+
+	page_data = (char *) kmap(page);
+	/*
+	 * Use vfs_read because some lower file systems don't have a
+	 * readpage method, and some file systems (esp. distributed ones)
+	 * don't like their pages to be accessed directly.  Using vfs_read
+	 * may be a little slower, but a lot safer, as the VFS does a lot of
+	 * the necessary magic for us.
+	 */
+	lower_file->f_pos = page_offset(page);
+	old_fs = get_fs();
+	set_fs(KERNEL_DS);
+	/*
+	 * generic_file_splice_write may call us on a file not opened for
+	 * reading, so temporarily allow reading.
+	 */
+	orig_mode = lower_file->f_mode;
+	lower_file->f_mode |= FMODE_READ;
+	err = vfs_read(lower_file, page_data, PAGE_CACHE_SIZE,
+		       &lower_file->f_pos);
+	lower_file->f_mode = orig_mode;
+	set_fs(old_fs);
+	if (err >= 0 && err < PAGE_CACHE_SIZE)
+		memset(page_data + err, 0, PAGE_CACHE_SIZE - err);
+	kunmap(page);
+
+	if (err < 0)
+		goto out;
+	err = 0;
+
+	/* if vfs_read succeeded above, sync up our times */
+	unionfs_copy_attr_times(inode);
+
+	flush_dcache_page(page);
+
+	/*
+	 * we have to unlock our page, b/c we _might_ have gotten a locked
+	 * page.  but we no longer have to wakeup on our page here, b/c
+	 * UnlockPage does it
+	 */
+out:
+	if (err == 0)
+		SetPageUptodate(page);
+	else
+		ClearPageUptodate(page);
+
+	unlock_page(page);
+	unionfs_check_file(file);
+	unionfs_read_unlock(file->f_path.dentry->d_sb);
+
+	return err;
+}
+
+static int unionfs_prepare_write(struct file *file, struct page *page,
+				 unsigned from, unsigned to)
+{
+	int err;
+
+	unionfs_read_lock(file->f_path.dentry->d_sb);
+	/*
+	 * This is the only place where we unconditionally copy the lower
+	 * attribute times before calling unionfs_file_revalidate.  The
+	 * reason is that our ->write calls do_sync_write which in turn will
+	 * call our ->prepare_write and then ->commit_write.  Before our
+	 * ->write is called, the lower mtimes are in sync, but by the time
+	 * the VFS calls our ->commit_write, the lower mtimes have changed.
+	 * Therefore, the only reasonable time for us to sync up from the
+	 * changed lower mtimes, and avoid an invariant violation warning,
+	 * is here, in ->prepare_write.
+	 */
+	unionfs_copy_attr_times(file->f_path.dentry->d_inode);
+	err = unionfs_file_revalidate(file, true);
+	unionfs_check_file(file);
+	unionfs_read_unlock(file->f_path.dentry->d_sb);
+
+	return err;
+}
+
+static int unionfs_commit_write(struct file *file, struct page *page,
+				unsigned from, unsigned to)
+{
+	int err = -ENOMEM;
+	struct inode *inode, *lower_inode;
+	struct file *lower_file = NULL;
+	unsigned bytes = to - from;
+	char *page_data = NULL;
+	mm_segment_t old_fs;
+
+	BUG_ON(file == NULL);
+
+	unionfs_read_lock(file->f_path.dentry->d_sb);
+	err = unionfs_file_revalidate(file, true);
+	if (unlikely(err))
+		goto out;
+	unionfs_check_file(file);
+
+	inode = page->mapping->host;
+	lower_inode = unionfs_lower_inode(inode);
+
+	if (UNIONFS_F(file) != NULL)
+		lower_file = unionfs_lower_file(file);
+
+	/* FIXME: is this assertion right here? */
+	BUG_ON(lower_file == NULL);
+
+	page_data = (char *)kmap(page);
+	lower_file->f_pos = page_offset(page) + from;
+
+	/*
+	 * We use vfs_write instead of copying page data and the
+	 * prepare_write/commit_write combo because file system's like
+	 * GFS/OCFS2 don't like things touching those directly,
+	 * calling the underlying write op, while a little bit slower, will
+	 * call all the FS specific code as well
+	 */
+	old_fs = get_fs();
+	set_fs(KERNEL_DS);
+	err = vfs_write(lower_file, page_data + from, bytes,
+			&lower_file->f_pos);
+	set_fs(old_fs);
+
+	kunmap(page);
+
+	if (err < 0)
+		goto out;
+
+	/* if vfs_write succeeded above, sync up our times/sizes */
+	fsstack_copy_inode_size(inode, lower_inode);
+	unionfs_copy_attr_times(inode);
+	mark_inode_dirty_sync(inode);
+
+out:
+	if (err < 0)
+		ClearPageUptodate(page);
+
+	unionfs_check_file(file);
+	unionfs_read_unlock(file->f_path.dentry->d_sb);
+	return err;		/* assume all is ok */
+}
+
+/*
+ * Although unionfs isn't a block-based file system, it may stack on one.
+ * ->bmap is needed, for example, to swapon(2) files.
+ */
+sector_t unionfs_bmap(struct address_space *mapping, sector_t block)
+{
+	int err = -EINVAL;
+	struct inode *inode, *lower_inode;
+	sector_t (*bmap)(struct address_space *, sector_t);
+
+	inode = (struct inode *)mapping->host;
+	lower_inode = unionfs_lower_inode(inode);
+	if (!lower_inode)
+		goto out;
+	bmap = lower_inode->i_mapping->a_ops->bmap;
+	if (bmap)
+		err = bmap(lower_inode->i_mapping, block);
+out:
+	return err;
+}
+
+
+struct address_space_operations unionfs_aops = {
+	.writepage	= unionfs_writepage,
+	.writepages	= unionfs_writepages,
+	.readpage	= unionfs_readpage,
+	.prepare_write	= unionfs_prepare_write,
+	.commit_write	= unionfs_commit_write,
+	.bmap		= unionfs_bmap,
+};
-- 
1.5.2.2

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