Hi,
below is the first version of the patch that reverses locking order of
page_lock and transaction start. I have tested it with fsx-linux, ltp DIO
tests etc. and lockdep didn't complain so hopefully I got it mostly right
but review is definitely needed. Especially I'd like to know what people
think about the way I've implemented ext3_page_mkwrite() - ext4 has
an incorrect code AFAICT because in ordered and journaled modes we should
write block of zeros and properly journal it (and no, block_page_mkwrite()
doesn't do it). We could implement ext3/4_page_mkwrite() in a similar way
we currently implement writepage calls but calling write_begin + write_end
does the job and should be only a tiny bit slower...
If nobody finds a serious flaw in the approach, I'll rediff the patch
against ext4 (I'll also try to convert delayed-alloc path - from a quick
look converting da_writepages path is going to be interesting).
I'm looking forward to your comments :)
Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR
---
Reverse locking order of page lock and transaction start in ext3 (i.e., page
lock now comes after the transaction start). Needed changes are:
1) Simply swap the order in ext3_write_begin() and ext3_..._write_end()
(allows removal of ext3_generic_write_end())
2) Implement ext3_page_mkwrite() to fill holes.
3) Change ext3_writeback_writepage() not to start a transaction at all,
ext3_ordered_writepage() starts a transaction only after unlocking
the page in block_write_full_page() (to attach buffers to the transaction),
ext3_journaled_writepage() gets references to buffers in the page, unlocks
the page and then starts a transaction.
Signed-off-by: Jan Kara <[email protected]>
---
fs/ext3/file.c | 19 ++++-
fs/ext3/inode.c | 236 +++++++++++++++++++++++++----------------------
include/linux/ext3_fs.h | 1 +
3 files changed, 145 insertions(+), 111 deletions(-)
diff --git a/fs/ext3/file.c b/fs/ext3/file.c
index acc4913..29fbdb6 100644
--- a/fs/ext3/file.c
+++ b/fs/ext3/file.c
@@ -106,6 +106,23 @@ force_commit:
return ret;
}
+static struct vm_operations_struct ext3_file_vm_ops = {
+ .fault = filemap_fault,
+ .page_mkwrite = ext3_page_mkwrite,
+};
+
+static int ext3_file_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ struct address_space *mapping = file->f_mapping;
+
+ if (!mapping->a_ops->readpage)
+ return -ENOEXEC;
+ file_accessed(file);
+ vma->vm_ops = &ext3_file_vm_ops;
+ vma->vm_flags |= VM_CAN_NONLINEAR;
+ return 0;
+}
+
const struct file_operations ext3_file_operations = {
.llseek = generic_file_llseek,
.read = do_sync_read,
@@ -116,7 +133,7 @@ const struct file_operations ext3_file_operations = {
#ifdef CONFIG_COMPAT
.compat_ioctl = ext3_compat_ioctl,
#endif
- .mmap = generic_file_mmap,
+ .mmap = ext3_file_mmap,
.open = generic_file_open,
.release = ext3_release_file,
.fsync = ext3_sync_file,
diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
index eb95670..210b886 100644
--- a/fs/ext3/inode.c
+++ b/fs/ext3/inode.c
@@ -1126,6 +1126,10 @@ static int walk_page_buffers( handle_t *handle,
* will _not_ run commit under these circumstances because handle->h_ref
* is elevated. We'll still have enough credits for the tiny quotafile
* write.
+ *
+ * Note that ext3_page_mkwrite can run write_begin / write_end without
+ * holding i_mutex (but holding non-exclusive i_alloc_sem to protect
+ * against truncates).
*/
static int do_journal_get_write_access(handle_t *handle,
struct buffer_head *bh)
@@ -1152,18 +1156,19 @@ static int ext3_write_begin(struct file *file, struct address_space *mapping,
to = from + len;
retry:
- page = __grab_cache_page(mapping, index);
- if (!page)
- return -ENOMEM;
- *pagep = page;
-
handle = ext3_journal_start(inode, needed_blocks);
if (IS_ERR(handle)) {
- unlock_page(page);
- page_cache_release(page);
ret = PTR_ERR(handle);
goto out;
}
+ page = __grab_cache_page(mapping, index);
+ if (!page) {
+ ext3_journal_stop(handle);
+ ret = -ENOMEM;
+ goto out;
+ }
+ *pagep = page;
+
ret = block_write_begin(file, mapping, pos, len, flags, pagep, fsdata,
ext3_get_block);
if (ret)
@@ -1175,8 +1180,8 @@ retry:
}
write_begin_failed:
if (ret) {
- ext3_journal_stop(handle);
unlock_page(page);
+ ext3_journal_stop(handle);
page_cache_release(page);
}
if (ret == -ENOSPC && ext3_should_retry_alloc(inode->i_sb, &retries))
@@ -1205,29 +1210,6 @@ static int write_end_fn(handle_t *handle, struct buffer_head *bh)
}
/*
- * Generic write_end handler for ordered and writeback ext3 journal modes.
- * We can't use generic_write_end, because that unlocks the page and we need to
- * unlock the page after ext3_journal_stop, but ext3_journal_stop must run
- * after block_write_end.
- */
-static int ext3_generic_write_end(struct file *file,
- struct address_space *mapping,
- loff_t pos, unsigned len, unsigned copied,
- struct page *page, void *fsdata)
-{
- struct inode *inode = file->f_mapping->host;
-
- copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
-
- if (pos+copied > inode->i_size) {
- i_size_write(inode, pos+copied);
- mark_inode_dirty(inode);
- }
-
- return copied;
-}
-
-/*
* We need to pick up the new inode size which generic_commit_write gave us
* `file' can be NULL - eg, when called from page_symlink().
*
@@ -1240,7 +1222,7 @@ static int ext3_ordered_write_end(struct file *file,
struct page *page, void *fsdata)
{
handle_t *handle = ext3_journal_current_handle();
- struct inode *inode = file->f_mapping->host;
+ struct inode *inode = mapping->host;
unsigned from, to;
int ret = 0, ret2;
@@ -1261,7 +1243,7 @@ static int ext3_ordered_write_end(struct file *file,
new_i_size = pos + copied;
if (new_i_size > EXT3_I(inode)->i_disksize)
EXT3_I(inode)->i_disksize = new_i_size;
- copied = ext3_generic_write_end(file, mapping, pos, len, copied,
+ copied = generic_write_end(file, mapping, pos, len, copied,
page, fsdata);
if (copied < 0)
ret = copied;
@@ -1269,8 +1251,6 @@ static int ext3_ordered_write_end(struct file *file,
ret2 = ext3_journal_stop(handle);
if (!ret)
ret = ret2;
- unlock_page(page);
- page_cache_release(page);
return ret ? ret : copied;
}
@@ -1281,7 +1261,7 @@ static int ext3_writeback_write_end(struct file *file,
struct page *page, void *fsdata)
{
handle_t *handle = ext3_journal_current_handle();
- struct inode *inode = file->f_mapping->host;
+ struct inode *inode = mapping->host;
int ret = 0, ret2;
loff_t new_i_size;
@@ -1289,7 +1269,7 @@ static int ext3_writeback_write_end(struct file *file,
if (new_i_size > EXT3_I(inode)->i_disksize)
EXT3_I(inode)->i_disksize = new_i_size;
- copied = ext3_generic_write_end(file, mapping, pos, len, copied,
+ copied = generic_write_end(file, mapping, pos, len, copied,
page, fsdata);
if (copied < 0)
ret = copied;
@@ -1297,8 +1277,6 @@ static int ext3_writeback_write_end(struct file *file,
ret2 = ext3_journal_stop(handle);
if (!ret)
ret = ret2;
- unlock_page(page);
- page_cache_release(page);
return ret ? ret : copied;
}
@@ -1337,10 +1315,10 @@ static int ext3_journalled_write_end(struct file *file,
ret = ret2;
}
+ unlock_page(page);
ret2 = ext3_journal_stop(handle);
if (!ret)
ret = ret2;
- unlock_page(page);
page_cache_release(page);
return ret ? ret : copied;
@@ -1418,11 +1396,10 @@ static int journal_dirty_data_fn(handle_t *handle, struct buffer_head *bh)
}
/*
- * Note that we always start a transaction even if we're not journalling
- * data. This is to preserve ordering: any hole instantiation within
- * __block_write_full_page -> ext3_get_block() should be journalled
- * along with the data so we don't crash and then get metadata which
- * refers to old data.
+ * Note that we don't need to start a transaction unless we're journaling
+ * data because we should have holes filled from ext3_page_mkwrite(). If
+ * we are journaling data, we cannot start transaction directly because
+ * transaction start ranks above page lock so we have to do some magic...
*
* In all journalling modes block_write_full_page() will start the I/O.
*
@@ -1466,8 +1443,6 @@ static int journal_dirty_data_fn(handle_t *handle, struct buffer_head *bh)
* disastrous. Any write() or metadata operation will sync the fs for
* us.
*
- * AKPM2: if all the page's buffers are mapped to disk and !data=journal,
- * we don't need to open a transaction here.
*/
static int ext3_ordered_writepage(struct page *page,
struct writeback_control *wbc)
@@ -1487,13 +1462,6 @@ static int ext3_ordered_writepage(struct page *page,
if (ext3_journal_current_handle())
goto out_fail;
- handle = ext3_journal_start(inode, ext3_writepage_trans_blocks(inode));
-
- if (IS_ERR(handle)) {
- ret = PTR_ERR(handle);
- goto out_fail;
- }
-
if (!page_has_buffers(page)) {
create_empty_buffers(page, inode->i_sb->s_blocksize,
(1 << BH_Dirty)|(1 << BH_Uptodate));
@@ -1517,16 +1485,21 @@ static int ext3_ordered_writepage(struct page *page,
* and generally junk.
*/
if (ret == 0) {
- err = walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE,
+ handle = ext3_journal_start(inode, ext3_writepage_trans_blocks(inode));
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out_put;
+ }
+
+ ret = walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE,
NULL, journal_dirty_data_fn);
+ err = ext3_journal_stop(handle);
if (!ret)
ret = err;
}
+out_put:
walk_page_buffers(handle, page_bufs, 0,
PAGE_CACHE_SIZE, NULL, bput_one);
- err = ext3_journal_stop(handle);
- if (!ret)
- ret = err;
return ret;
out_fail:
@@ -1539,27 +1512,16 @@ static int ext3_writeback_writepage(struct page *page,
struct writeback_control *wbc)
{
struct inode *inode = page->mapping->host;
- handle_t *handle = NULL;
int ret = 0;
- int err;
if (ext3_journal_current_handle())
goto out_fail;
- handle = ext3_journal_start(inode, ext3_writepage_trans_blocks(inode));
- if (IS_ERR(handle)) {
- ret = PTR_ERR(handle);
- goto out_fail;
- }
-
if (test_opt(inode->i_sb, NOBH) && ext3_should_writeback_data(inode))
ret = nobh_writepage(page, ext3_get_block, wbc);
else
ret = block_write_full_page(page, ext3_get_block, wbc);
- err = ext3_journal_stop(handle);
- if (!ret)
- ret = err;
return ret;
out_fail:
@@ -1571,7 +1533,9 @@ out_fail:
static int ext3_journalled_writepage(struct page *page,
struct writeback_control *wbc)
{
- struct inode *inode = page->mapping->host;
+ struct address_space *mapping = page->mapping;
+ struct inode *inode = mapping->host;
+ struct buffer_head *page_bufs;
handle_t *handle = NULL;
int ret = 0;
int err;
@@ -1579,12 +1543,6 @@ static int ext3_journalled_writepage(struct page *page,
if (ext3_journal_current_handle())
goto no_write;
- handle = ext3_journal_start(inode, ext3_writepage_trans_blocks(inode));
- if (IS_ERR(handle)) {
- ret = PTR_ERR(handle);
- goto no_write;
- }
-
if (!page_has_buffers(page) || PageChecked(page)) {
/*
* It's mmapped pagecache. Add buffers and journal it. There
@@ -1593,19 +1551,37 @@ static int ext3_journalled_writepage(struct page *page,
ClearPageChecked(page);
ret = block_prepare_write(page, 0, PAGE_CACHE_SIZE,
ext3_get_block);
- if (ret != 0) {
- ext3_journal_stop(handle);
+ if (ret != 0)
goto out_unlock;
+
+ page_bufs = page_buffers(page);
+ walk_page_buffers(handle, page_bufs, 0,
+ PAGE_CACHE_SIZE, NULL, bget_one);
+ /* As soon as we unlock the page, it can go away, but we have
+ * references to buffers so we are safe */
+ unlock_page(page);
+
+ handle = ext3_journal_start(inode,
+ ext3_writepage_trans_blocks(inode));
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out;
}
- ret = walk_page_buffers(handle, page_buffers(page), 0,
+
+ ret = walk_page_buffers(handle, page_bufs, 0,
PAGE_CACHE_SIZE, NULL, do_journal_get_write_access);
- err = walk_page_buffers(handle, page_buffers(page), 0,
+ err = walk_page_buffers(handle, page_bufs, 0,
PAGE_CACHE_SIZE, NULL, write_end_fn);
if (ret == 0)
ret = err;
+ err = ext3_journal_stop(handle);
+ if (!ret)
+ ret = err;
+
+ walk_page_buffers(handle, page_bufs, 0,
+ PAGE_CACHE_SIZE, NULL, bput_one);
EXT3_I(inode)->i_state |= EXT3_STATE_JDATA;
- unlock_page(page);
} else {
/*
* It may be a page full of checkpoint-mode buffers. We don't
@@ -1614,9 +1590,6 @@ static int ext3_journalled_writepage(struct page *page,
*/
ret = block_write_full_page(page, ext3_get_block, wbc);
}
- err = ext3_journal_stop(handle);
- if (!ret)
- ret = err;
out:
return ret;
@@ -1821,7 +1794,7 @@ void ext3_set_aops(struct inode *inode)
* This required during truncate. We need to physically zero the tail end
* of that block so it doesn't yield old data if the file is later grown.
*/
-static int ext3_block_truncate_page(handle_t *handle, struct page *page,
+static int ext3_block_truncate_page(handle_t *handle,
struct address_space *mapping, loff_t from)
{
ext3_fsblk_t index = from >> PAGE_CACHE_SHIFT;
@@ -1829,8 +1802,13 @@ static int ext3_block_truncate_page(handle_t *handle, struct page *page,
unsigned blocksize, iblock, length, pos;
struct inode *inode = mapping->host;
struct buffer_head *bh;
+ struct page *page;
int err = 0;
+ page = grab_cache_page(mapping, from >> PAGE_CACHE_SHIFT);
+ if (!page)
+ return -EINVAL;
+
blocksize = inode->i_sb->s_blocksize;
length = blocksize - (offset & (blocksize - 1));
iblock = index << (PAGE_CACHE_SHIFT - inode->i_sb->s_blocksize_bits);
@@ -2293,7 +2271,6 @@ void ext3_truncate(struct inode *inode)
int n;
long last_block;
unsigned blocksize = inode->i_sb->s_blocksize;
- struct page *page;
if (!(S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode) ||
S_ISLNK(inode->i_mode)))
@@ -2303,36 +2280,16 @@ void ext3_truncate(struct inode *inode)
if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
return;
- /*
- * We have to lock the EOF page here, because lock_page() nests
- * outside journal_start().
- */
- if ((inode->i_size & (blocksize - 1)) == 0) {
- /* Block boundary? Nothing to do */
- page = NULL;
- } else {
- page = grab_cache_page(mapping,
- inode->i_size >> PAGE_CACHE_SHIFT);
- if (!page)
- return;
- }
-
handle = start_transaction(inode);
- if (IS_ERR(handle)) {
- if (page) {
- clear_highpage(page);
- flush_dcache_page(page);
- unlock_page(page);
- page_cache_release(page);
- }
+ if (IS_ERR(handle))
return; /* AKPM: return what? */
- }
last_block = (inode->i_size + blocksize-1)
>> EXT3_BLOCK_SIZE_BITS(inode->i_sb);
- if (page)
- ext3_block_truncate_page(handle, page, mapping, inode->i_size);
+ if (inode->i_size & (blocksize - 1))
+ if (ext3_block_truncate_page(handle, mapping, inode->i_size))
+ goto out_stop;
n = ext3_block_to_path(inode, last_block, offsets, NULL);
if (n == 0)
@@ -3306,3 +3263,62 @@ int ext3_change_inode_journal_flag(struct inode *inode, int val)
return err;
}
+
+static int ext3_bh_mapped(handle_t *handle, struct buffer_head *bh)
+{
+ return !buffer_mapped(bh);
+}
+
+int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page)
+{
+ struct file *file = vma->vm_file;
+ struct inode *inode = file->f_path.dentry->d_inode;
+ struct address_space *mapping = inode->i_mapping;
+ unsigned long len;
+ loff_t size;
+ int ret = -EINVAL;
+
+ /*
+ * Get i_alloc_sem to stop truncates messing with the inode. We cannot
+ * get i_mutex because we are already holding mmap_sem. This makes
+ * it possible for write_begin and write_end to run concurrently
+ * on a single file (not on a single page because of page_lock).
+ * We seem to handle this just fine...
+ */
+ down_read(&inode->i_alloc_sem);
+ size = i_size_read(inode);
+ if (page->mapping != mapping || size <= page_offset(page)
+ || !PageUptodate(page)) {
+ /* page got truncated from under us? */
+ goto out_unlock;
+ }
+ ret = 0;
+ if (PageMappedToDisk(page))
+ goto out_unlock;
+
+ if (page->index == size >> PAGE_CACHE_SHIFT)
+ len = size & ~PAGE_CACHE_MASK;
+ else
+ len = PAGE_CACHE_SIZE;
+
+ if (page_has_buffers(page)) {
+ if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
+ ext3_bh_mapped))
+ goto out_unlock;
+ }
+
+ /* OK, we need to fill the hole... We simply write the page. */
+ printk(KERN_INFO "Writing page %lu of ino %lu\n", page->index, inode->i_ino);
+ ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),
+ len, AOP_FLAG_UNINTERRUPTIBLE, &page, NULL);
+ if (ret < 0)
+ goto out_unlock;
+ ret = mapping->a_ops->write_end(file, mapping, page_offset(page), len,
+ len, page, NULL);
+ if (ret < 0)
+ goto out_unlock;
+ ret = 0;
+out_unlock:
+ up_read(&inode->i_alloc_sem);
+ return ret;
+}
diff --git a/include/linux/ext3_fs.h b/include/linux/ext3_fs.h
index 36c5403..715c35e 100644
--- a/include/linux/ext3_fs.h
+++ b/include/linux/ext3_fs.h
@@ -836,6 +836,7 @@ extern void ext3_truncate (struct inode *);
extern void ext3_set_inode_flags(struct inode *);
extern void ext3_get_inode_flags(struct ext3_inode_info *);
extern void ext3_set_aops(struct inode *inode);
+extern int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page);
/* ioctl.c */
extern int ext3_ioctl (struct inode *, struct file *, unsigned int,
--
1.5.2.4
On Thu, Mar 27, 2008 at 05:27:42PM +0100, Jan Kara wrote:
> Hi,
>
> below is the first version of the patch that reverses locking order of
> page_lock and transaction start. I have tested it with fsx-linux, ltp DIO
> tests etc. and lockdep didn't complain so hopefully I got it mostly right
> but review is definitely needed. Especially I'd like to know what people
> think about the way I've implemented ext3_page_mkwrite() - ext4 has
> an incorrect code AFAICT because in ordered and journaled modes we should
> write block of zeros and properly journal it (and no, block_page_mkwrite()
> doesn't do it). We could implement ext3/4_page_mkwrite() in a similar way
> we currently implement writepage calls but calling write_begin + write_end
> does the job and should be only a tiny bit slower...
> If nobody finds a serious flaw in the approach, I'll rediff the patch
> against ext4 (I'll also try to convert delayed-alloc path - from a quick
> look converting da_writepages path is going to be interesting).
> I'm looking forward to your comments :)
>
> Honza
> --
> Jan Kara <[email protected]>
> SUSE Labs, CR
>
> ---
>
> Reverse locking order of page lock and transaction start in ext3 (i.e., page
> lock now comes after the transaction start). Needed changes are:
> 1) Simply swap the order in ext3_write_begin() and ext3_..._write_end()
> (allows removal of ext3_generic_write_end())
> 2) Implement ext3_page_mkwrite() to fill holes.
> 3) Change ext3_writeback_writepage() not to start a transaction at all,
> ext3_ordered_writepage() starts a transaction only after unlocking
> the page in block_write_full_page() (to attach buffers to the transaction),
> ext3_journaled_writepage() gets references to buffers in the page, unlocks
> the page and then starts a transaction.
>
> Signed-off-by: Jan Kara <[email protected]>
>
> ---
> fs/ext3/file.c | 19 ++++-
> fs/ext3/inode.c | 236 +++++++++++++++++++++++++----------------------
> include/linux/ext3_fs.h | 1 +
> 3 files changed, 145 insertions(+), 111 deletions(-)
>
....
> +
> +static int ext3_bh_mapped(handle_t *handle, struct buffer_head *bh)
> +{
> + return !buffer_mapped(bh);
> +}
> +
> +int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> +{
> + struct file *file = vma->vm_file;
> + struct inode *inode = file->f_path.dentry->d_inode;
> + struct address_space *mapping = inode->i_mapping;
> + unsigned long len;
> + loff_t size;
> + int ret = -EINVAL;
> +
> + /*
> + * Get i_alloc_sem to stop truncates messing with the inode. We cannot
> + * get i_mutex because we are already holding mmap_sem. This makes
> + * it possible for write_begin and write_end to run concurrently
> + * on a single file (not on a single page because of page_lock).
> + * We seem to handle this just fine...
> + */
> + down_read(&inode->i_alloc_sem);
> + size = i_size_read(inode);
> + if (page->mapping != mapping || size <= page_offset(page)
> + || !PageUptodate(page)) {
> + /* page got truncated from under us? */
> + goto out_unlock;
> + }
> + ret = 0;
> + if (PageMappedToDisk(page))
> + goto out_unlock;
> +
> + if (page->index == size >> PAGE_CACHE_SHIFT)
> + len = size & ~PAGE_CACHE_MASK;
> + else
> + len = PAGE_CACHE_SIZE;
> +
> + if (page_has_buffers(page)) {
> + if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
> + ext3_bh_mapped))
> + goto out_unlock;
> + }
> +
> + /* OK, we need to fill the hole... We simply write the page. */
> + printk(KERN_INFO "Writing page %lu of ino %lu\n", page->index, inode->i_ino);
> + ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),
> + len, AOP_FLAG_UNINTERRUPTIBLE, &page, NULL);
> + if (ret < 0)
> + goto out_unlock;
> + ret = mapping->a_ops->write_end(file, mapping, page_offset(page), len,
> + len, page, NULL);
> + if (ret < 0)
> + goto out_unlock;
> + ret = 0;
> +out_unlock:
> + up_read(&inode->i_alloc_sem);
> + return ret;
> +}
> diff --git a/include/linux/ext3_fs.h b/include/linux/ext3_fs.h
> index 36c5403..715c35e 100644
> --- a/include/linux/ext3_fs.h
> +++ b/include/linux/ext3_fs.h
> @@ -836,6 +836,7 @@ extern void ext3_truncate (struct inode *);
> extern void ext3_set_inode_flags(struct inode *);
> extern void ext3_get_inode_flags(struct ext3_inode_info *);
> extern void ext3_set_aops(struct inode *inode);
> +extern int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page);
>
> /* ioctl.c */
> extern int ext3_ioctl (struct inode *, struct file *, unsigned int,
The comments on block_page_mkwrite says taking a lock on page protect it
against truncate. Why do we need to take i_alloc_sem ? Is it because
after changing the locking order we can't any more take the page lock
here because we need to take it after the transaction is started ?
My patch to use page_mkwrite on ext3 resulted in this discussion.
http://article.gmane.org/gmane.comp.file-systems.ext4/5731
Does the above means that it will call page_mkwrite with page lock held.
That would imply that we can't start transaction inside page_mkwrite
Why do you think that current Ext4 code page_mkwrite is wrong ?
We just need to reserve space for the page we are dirtying right.
I have tried a similar change and later dropped it because we didn't
had much anything to journal
http://article.gmane.org/gmane.comp.file-systems.ext4/5201
This had the inode_lock taken which lockdep complained about.
ext4_get_blocks create a journal handle for all meta update if we don't
have one.
-aneesh
Hello,
> On Thu, Mar 27, 2008 at 05:27:42PM +0100, Jan Kara wrote:
> > Hi,
> >
> > below is the first version of the patch that reverses locking order of
> > page_lock and transaction start. I have tested it with fsx-linux, ltp DIO
> > tests etc. and lockdep didn't complain so hopefully I got it mostly right
> > but review is definitely needed. Especially I'd like to know what people
> > think about the way I've implemented ext3_page_mkwrite() - ext4 has
> > an incorrect code AFAICT because in ordered and journaled modes we should
> > write block of zeros and properly journal it (and no, block_page_mkwrite()
> > doesn't do it). We could implement ext3/4_page_mkwrite() in a similar way
> > we currently implement writepage calls but calling write_begin + write_end
> > does the job and should be only a tiny bit slower...
> > If nobody finds a serious flaw in the approach, I'll rediff the patch
> > against ext4 (I'll also try to convert delayed-alloc path - from a quick
> > look converting da_writepages path is going to be interesting).
> > I'm looking forward to your comments :)
> >
> > Honza
> > --
> > Jan Kara <[email protected]>
> > SUSE Labs, CR
> >
> > ---
> >
> > Reverse locking order of page lock and transaction start in ext3 (i.e., page
> > lock now comes after the transaction start). Needed changes are:
> > 1) Simply swap the order in ext3_write_begin() and ext3_..._write_end()
> > (allows removal of ext3_generic_write_end())
> > 2) Implement ext3_page_mkwrite() to fill holes.
> > 3) Change ext3_writeback_writepage() not to start a transaction at all,
> > ext3_ordered_writepage() starts a transaction only after unlocking
> > the page in block_write_full_page() (to attach buffers to the transaction),
> > ext3_journaled_writepage() gets references to buffers in the page, unlocks
> > the page and then starts a transaction.
> >
> > Signed-off-by: Jan Kara <[email protected]>
> >
> > ---
> > fs/ext3/file.c | 19 ++++-
> > fs/ext3/inode.c | 236 +++++++++++++++++++++++++----------------------
> > include/linux/ext3_fs.h | 1 +
> > 3 files changed, 145 insertions(+), 111 deletions(-)
> >
>
> ....
>
> > +
> > +static int ext3_bh_mapped(handle_t *handle, struct buffer_head *bh)
> > +{
> > + return !buffer_mapped(bh);
> > +}
> > +
> > +int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> > +{
> > + struct file *file = vma->vm_file;
> > + struct inode *inode = file->f_path.dentry->d_inode;
> > + struct address_space *mapping = inode->i_mapping;
> > + unsigned long len;
> > + loff_t size;
> > + int ret = -EINVAL;
> > +
> > + /*
> > + * Get i_alloc_sem to stop truncates messing with the inode. We cannot
> > + * get i_mutex because we are already holding mmap_sem. This makes
> > + * it possible for write_begin and write_end to run concurrently
> > + * on a single file (not on a single page because of page_lock).
> > + * We seem to handle this just fine...
> > + */
> > + down_read(&inode->i_alloc_sem);
> > + size = i_size_read(inode);
> > + if (page->mapping != mapping || size <= page_offset(page)
> > + || !PageUptodate(page)) {
> > + /* page got truncated from under us? */
> > + goto out_unlock;
> > + }
> > + ret = 0;
> > + if (PageMappedToDisk(page))
> > + goto out_unlock;
> > +
> > + if (page->index == size >> PAGE_CACHE_SHIFT)
> > + len = size & ~PAGE_CACHE_MASK;
> > + else
> > + len = PAGE_CACHE_SIZE;
> > +
> > + if (page_has_buffers(page)) {
> > + if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
> > + ext3_bh_mapped))
> > + goto out_unlock;
> > + }
> > +
> > + /* OK, we need to fill the hole... We simply write the page. */
> > + printk(KERN_INFO "Writing page %lu of ino %lu\n", page->index, inode->i_ino);
> > + ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),
> > + len, AOP_FLAG_UNINTERRUPTIBLE, &page, NULL);
> > + if (ret < 0)
> > + goto out_unlock;
> > + ret = mapping->a_ops->write_end(file, mapping, page_offset(page), len,
> > + len, page, NULL);
> > + if (ret < 0)
> > + goto out_unlock;
> > + ret = 0;
> > +out_unlock:
> > + up_read(&inode->i_alloc_sem);
> > + return ret;
> > +}
> > diff --git a/include/linux/ext3_fs.h b/include/linux/ext3_fs.h
> > index 36c5403..715c35e 100644
> > --- a/include/linux/ext3_fs.h
> > +++ b/include/linux/ext3_fs.h
> > @@ -836,6 +836,7 @@ extern void ext3_truncate (struct inode *);
> > extern void ext3_set_inode_flags(struct inode *);
> > extern void ext3_get_inode_flags(struct ext3_inode_info *);
> > extern void ext3_set_aops(struct inode *inode);
> > +extern int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page);
> >
> > /* ioctl.c */
> > extern int ext3_ioctl (struct inode *, struct file *, unsigned int,
>
> The comments on block_page_mkwrite says taking a lock on page protect it
> against truncate. Why do we need to take i_alloc_sem ? Is it because
> after changing the locking order we can't any more take the page lock
> here because we need to take it after the transaction is started ?
Well, there are few things which lead to the locking I do:
1) We need to issue data write - actually comments in
http://article.gmane.org/gmane.comp.file-systems.ext4/5201 are partly
incorrect. For ordered and journal data mode we must write zeros to the
file if we fill in holes (which is what should be in the page) -
otherwise if we crash before the writepage call (which can go to the
next transaction), we would see uninitialized data in the file.
2) I didn't want to reimplement writepage calls for writeback, ordered
and journal so I've used write_begin and write_end calls. These calls
take page_lock themselves so we cannot take it in page_mkwrite().
3) We need to be protected against truncate (normally, i_mutex does this
but we cannot take it in page_mkwrite() due to lock ordering issues). So
we use i_alloc_sem for this.
But looking at the code with a fresh look (it's always good to look at
the code a week after you've written it whether you still like it ;),
I think I could try to move writepage() code into helper functions
and use these functions from writepage() and page_mkwrite(). That
wouldn't have problems with code duplication and the result would be
hopefully nicer than it is now.
> My patch to use page_mkwrite on ext3 resulted in this discussion.
> http://article.gmane.org/gmane.comp.file-systems.ext4/5731
Thanks for the pointer.
> Does the above means that it will call page_mkwrite with page lock held.
> That would imply that we can't start transaction inside page_mkwrite
We could always unlock the page and then lock it again. But other
journaling filesystems (e.g. OCFS2) also need to start a transaction in
page_mkwrite so I'm not sure the change Nick suggests will ever happen.
> Why do you think that current Ext4 code page_mkwrite is wrong ?
> We just need to reserve space for the page we are dirtying right.
Well, that gets rid of the ENOSPC problem but as I explain above, you
also need to write zeros to the file in can you allocate a new block.
> I have tried a similar change and later dropped it because we didn't
> had much anything to journal
> http://article.gmane.org/gmane.comp.file-systems.ext4/5201
> This had the inode_lock taken which lockdep complained about.
>
> ext4_get_blocks create a journal handle for all meta update if we don't
> have one.
Yes, but when we reverse the lock ordering of page_lock and
transaction start, we have to start a transaction before we take the
page_lock... So ext4_get_blocks() is too late to start a transaction at
least for page_mkwrite().
Honza
--
Jan Kara <[email protected]>
SuSE CR Labs
On Thu, Mar 27, 2008 at 05:27:42PM +0100, Jan Kara wrote:
> Hi,
>
> below is the first version of the patch that reverses locking order of
> page_lock and transaction start. I have tested it with fsx-linux, ltp DIO
> tests etc. and lockdep didn't complain so hopefully I got it mostly right
> but review is definitely needed. Especially I'd like to know what people
> think about the way I've implemented ext3_page_mkwrite() - ext4 has
> an incorrect code AFAICT because in ordered and journaled modes we should
> write block of zeros and properly journal it (and no, block_page_mkwrite()
> doesn't do it). We could implement ext3/4_page_mkwrite() in a similar way
> we currently implement writepage calls but calling write_begin + write_end
> does the job and should be only a tiny bit slower...
> If nobody finds a serious flaw in the approach, I'll rediff the patch
> against ext4 (I'll also try to convert delayed-alloc path - from a quick
> look converting da_writepages path is going to be interesting).
> I'm looking forward to your comments :)
>
I guess we should make this change first to Ext4 and then make the
same to ext3.
-aneesh