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SubjectRe: [patch 2/5] fs: introduce new aops and infrastructure
Hi Nick,

On Wed, Mar 14, 2007 at 02:38:22PM +0100, Nick Piggin wrote:
> Introduce write_begin, write_end, and perform_write aops.
>
> These are intended to replace prepare_write and commit_write with more
> flexible alternatives that are also able to avoid the buffered write
> deadlock problems efficiently (which prepare_write is unable to do).

> Index: linux-2.6/include/linux/fs.h
> ===================================================================
> --- linux-2.6.orig/include/linux/fs.h
> +++ linux-2.6/include/linux/fs.h
> @@ -449,6 +449,17 @@ struct address_space_operations {
> */
> int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
> int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
> +
> + int (*write_begin)(struct file *, struct address_space *mapping,
> + loff_t pos, unsigned len, int intr,
> + struct page **pagep, void **fsdata);
> + int (*write_end)(struct file *, struct address_space *mapping,
> + loff_t pos, unsigned len, unsigned copied,
> + struct page *page, void *fsdata);

Are we going to get rid of the file and intr arguments btw? I'm not sure
intr is useful, and mapping is probably enough to get whatever we inside
->write_begin / ->write_end.

Also, I noticed that you didn't export block_write_begin(),
simple_write_begin(), block_write_end() and simple_write_end() - I think we
want those for client modules.


Attached is a quick patch to hook up the existing ocfs2 write code. This has
been compile tested only for now - one of my test machines isn't
cooperating, so a runtime test will have to wait until tommorrow.

One interesting side effect is that we no longer pass AOP_TRUNCATE_PAGE up a
level. This gives callers less to deal with. And it means that ocfs2 doesn't
have to use the ocfs2_*_lock_with_page() cluster lock variants in
ocfs2_block_write_begin() because it can order cluster locks outside of the
page lock there.

My ocfs2 write rework will be a more serious user of these stuff, including
the fsdata backpointer. That code will also eliminate the entire
ocfs2_*_lock_with_page() cluster locking workarounds for write (they'll have
to remain for ->readpage()). I'm beginning work on cleaning those ocfs2
patches up and getting them plugged into this stuff. I hope to post them in
the next day or two.
--Mark

--
Mark Fasheh
Senior Software Developer, Oracle
mark.fasheh@oracle.com

ocfs2: Convert to new aops

Turn ocfs2_prepare_write() and ocfs2_commit_write() into ocfs2_write_begin()
and ocfs2_write_end().

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
index 93628b0..e7bcbbd 100644
--- a/fs/ocfs2/aops.c
+++ b/fs/ocfs2/aops.c
@@ -293,29 +293,30 @@ int ocfs2_prepare_write_nolock(struct in
}

/*
- * ocfs2_prepare_write() can be an outer-most ocfs2 call when it is called
- * from loopback. It must be able to perform its own locking around
- * ocfs2_get_block().
+ * ocfs2_write_begin() can be an outer-most ocfs2 call when it is
+ * called from elsewhere in the kernel. It must be able to perform its
+ * own locking around ocfs2_get_block().
*/
-static int ocfs2_prepare_write(struct file *file, struct page *page,
- unsigned from, unsigned to)
+static int ocfs2_write_begin(struct file *file, struct address_space *mapping,
+ loff_t pos, unsigned len, int intr,
+ struct page **pagep, void **fsdata)
{
- struct inode *inode = page->mapping->host;
+ struct inode *inode = mapping->host;
int ret;

- mlog_entry("(0x%p, 0x%p, %u, %u)\n", file, page, from, to);
-
- ret = ocfs2_meta_lock_with_page(inode, NULL, 0, page);
+ ret = ocfs2_meta_lock(inode, NULL, 0);
if (ret != 0) {
mlog_errno(ret);
goto out;
}

- ret = ocfs2_prepare_write_nolock(inode, page, from, to);
+ down_read(&OCFS2_I(inode)->ip_alloc_sem);
+ ret = block_write_begin(file, mapping, pos, len, intr, pagep, fsdata,
+ ocfs2_get_block);
+ up_read(&OCFS2_I(inode)->ip_alloc_sem);

ocfs2_meta_unlock(inode, 0);
out:
- mlog_exit(ret);
return ret;
}

@@ -388,16 +389,21 @@ out:
return handle;
}

-static int ocfs2_commit_write(struct file *file, struct page *page,
- unsigned from, unsigned to)
+static int ocfs2_write_end(struct file *file, struct address_space *mapping,
+ loff_t pos, unsigned len, unsigned copied,
+ struct page *page, void *fsdata)
{
int ret;
+ unsigned from, to;
struct buffer_head *di_bh = NULL;
struct inode *inode = page->mapping->host;
handle_t *handle = NULL;
struct ocfs2_dinode *di;

- mlog_entry("(0x%p, 0x%p, %u, %u)\n", file, page, from, to);
+ mlog_entry("(0x%p, 0x%p)\n", file, page);
+
+ from = pos & (PAGE_CACHE_SIZE - 1);
+ to = from + len;

/* NOTE: ocfs2_file_aio_write has ensured that it's safe for
* us to continue here without rechecking the I/O against
@@ -441,8 +447,10 @@ static int ocfs2_commit_write(struct fil
}

/* might update i_size */
- ret = generic_commit_write(file, page, from, to);
- if (ret < 0) {
+ copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
+ if (copied < 0) {
+ ret = copied;
+ copied = 0;
mlog_errno(ret);
goto out_commit;
}
@@ -458,11 +466,10 @@ static int ocfs2_commit_write(struct fil
di->i_size = cpu_to_le64((u64)i_size_read(inode));

ret = ocfs2_journal_dirty(handle, di_bh);
- if (ret < 0) {
+ if (ret < 0)
mlog_errno(ret);
- goto out_commit;
- }

+ ret = 0;
out_commit:
ocfs2_commit_trans(OCFS2_SB(inode->i_sb), handle);
out_unlock_data:
@@ -470,11 +477,29 @@ out_unlock_data:
out_unlock_meta:
ocfs2_meta_unlock(inode, 1);
out:
+ if (ret && ret != AOP_TRUNCATED_PAGE) {
+ /*
+ * We caught an error before block_write_end() -
+ * unlock and free the page.
+ */
+ unlock_page(page);
+ page_cache_release(page);
+ } else if (ret == AOP_TRUNCATED_PAGE) {
+ /*
+ * The cluster locking code avoided a deadlock by
+ * unlocking the page - simply release here and turn
+ * the error into something negative for the caller to
+ * key on.
+ */
+ page_cache_release(page);
+ ret = -EAGAIN;
+ }
+
if (di_bh)
brelse(di_bh);

mlog_exit(ret);
- return ret;
+ return copied ? copied : ret;
}

static sector_t ocfs2_bmap(struct address_space *mapping, sector_t block)
@@ -657,8 +682,8 @@ out:
const struct address_space_operations ocfs2_aops = {
.readpage = ocfs2_readpage,
.writepage = ocfs2_writepage,
- .prepare_write = ocfs2_prepare_write,
- .commit_write = ocfs2_commit_write,
+ .write_begin = ocfs2_write_begin,
+ .write_end = ocfs2_write_end,
.bmap = ocfs2_bmap,
.sync_page = block_sync_page,
.direct_IO = ocfs2_direct_IO
-
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