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ext4: simplify io_end handling for AIO DIO

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When mapping blocks for direct IO, we allocate io_end structure before
mapping blocks and store pointer to it in the inode. This creates a
requirement that any AIO DIO using io_end must be protected by i_mutex.
This created problems in the past with dioread_nolock mode which was
corrupting io_end pointers. Also io_end is allocated unnecessarily in
case where we don't need to convert any extents (which is a common case
for example when overwriting file).

We fix the problem by allocating io_end only once we return unwritten
extent from block mapping function for AIO DIO (so we can save some
pointless io_end allocations) and we pass pointer to it in bh->b_private
which generic DIO code later passes to our end IO callback. That way we
remove any need for global pointer to io_end structure and thus fix the
races.

The downside of this change is that the checking for unwritten IO in
flight in ext4_extents_can_be_merged() is more racy since we now
increment i_unwritten / set EXT4_STATE_DIO_UNWRITTEN only after dropping
i_data_sem. However the check has been racy already before because
ext4_writepages() already increment i_unwritten after dropping
i_data_sem and reserved blocks save us from hitting ENOSPC in the worst
case.

Signed-off-by: Jan Kara <jack@suse.cz>
This commit is contained in:
Jan Kara
2016-03-08 23:36:46 -05:00
committed by Theodore Ts'o
parent efe70c2951
commit 109811c20f
3 changed files with 74 additions and 104 deletions
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133
fs/ext4/inode.c
View File

@@ -797,18 +797,16 @@ int ext4_dio_get_block(struct inode *inode, sector_t iblock,
}
/*
* Get block function for DIO writes when we create unwritten extent if
* Get block function for AIO DIO writes when we create unwritten extent if
* blocks are not allocated yet. The extent will be converted to written
* after IO is complete.
*/
static int ext4_dio_get_block_unwritten(struct inode *inode, sector_t iblock,
struct buffer_head *bh_result, int create)
static int ext4_dio_get_block_unwritten_async(struct inode *inode,
sector_t iblock, struct buffer_head *bh_result, int create)
{
handle_t *handle;
int ret;
ext4_debug("ext4_dio_get_block_unwritten: inode %lu, create flag %d\n",
inode->i_ino, create);
/* We don't expect handle for direct IO */
WARN_ON_ONCE(ext4_journal_current_handle());
@@ -818,16 +816,62 @@ static int ext4_dio_get_block_unwritten(struct inode *inode, sector_t iblock,
ret = _ext4_get_block(inode, iblock, bh_result,
EXT4_GET_BLOCKS_IO_CREATE_EXT);
ext4_journal_stop(handle);
if (!ret && buffer_unwritten(bh_result)) {
ext4_io_end_t *io_end = ext4_inode_aio(inode);
/*
* When doing DIO using unwritten extents, we need io_end to convert
* unwritten extents to written on IO completion. We allocate io_end
* once we spot unwritten extent and store it in b_private. Generic
* DIO code keeps b_private set and furthermore passes the value to
* our completion callback in 'private' argument.
*/
if (!ret && buffer_unwritten(bh_result)) {
if (!bh_result->b_private) {
ext4_io_end_t *io_end;
io_end = ext4_init_io_end(inode, GFP_KERNEL);
if (!io_end)
return -ENOMEM;
bh_result->b_private = io_end;
ext4_set_io_unwritten_flag(inode, io_end);
}
set_buffer_defer_completion(bh_result);
WARN_ON_ONCE(io_end && !(io_end->flag & EXT4_IO_END_UNWRITTEN));
}
return ret;
}
/*
* Get block function for non-AIO DIO writes when we create unwritten extent if
* blocks are not allocated yet. The extent will be converted to written
* after IO is complete from ext4_ext_direct_IO() function.
*/
static int ext4_dio_get_block_unwritten_sync(struct inode *inode,
sector_t iblock, struct buffer_head *bh_result, int create)
{
handle_t *handle;
int ret;
/* We don't expect handle for direct IO */
WARN_ON_ONCE(ext4_journal_current_handle());
handle = start_dio_trans(inode, bh_result);
if (IS_ERR(handle))
return PTR_ERR(handle);
ret = _ext4_get_block(inode, iblock, bh_result,
EXT4_GET_BLOCKS_IO_CREATE_EXT);
ext4_journal_stop(handle);
/*
* Mark inode as having pending DIO writes to unwritten extents.
* ext4_ext_direct_IO() checks this flag and converts extents to
* written.
*/
if (!ret && buffer_unwritten(bh_result))
ext4_set_inode_state(inode, EXT4_STATE_DIO_UNWRITTEN);
return ret;
}
static int ext4_dio_get_block_overwrite(struct inode *inode, sector_t iblock,
struct buffer_head *bh_result, int create)
{
@@ -3243,7 +3287,7 @@ out:
static void ext4_end_io_dio(struct kiocb *iocb, loff_t offset,
ssize_t size, void *private)
{
ext4_io_end_t *io_end = iocb->private;
ext4_io_end_t *io_end = private;
/* if not async direct IO just return */
if (!io_end)
@@ -3251,10 +3295,8 @@ static void ext4_end_io_dio(struct kiocb *iocb, loff_t offset,
ext_debug("ext4_end_io_dio(): io_end 0x%p "
"for inode %lu, iocb 0x%p, offset %llu, size %zd\n",
iocb->private, io_end->inode->i_ino, iocb, offset,
size);
io_end, io_end->inode->i_ino, iocb, offset, size);
iocb->private = NULL;
io_end->offset = offset;
io_end->size = size;
ext4_put_io_end(io_end);
@@ -3290,7 +3332,6 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
get_block_t *get_block_func = NULL;
int dio_flags = 0;
loff_t final_size = offset + count;
ext4_io_end_t *io_end = NULL;
/* Use the old path for reads and writes beyond i_size. */
if (iov_iter_rw(iter) != WRITE || final_size > inode->i_size)
@@ -3315,16 +3356,17 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
/*
* We could direct write to holes and fallocate.
*
* Allocated blocks to fill the hole are marked as
* unwritten to prevent parallel buffered read to expose
* the stale data before DIO complete the data IO.
* Allocated blocks to fill the hole are marked as unwritten to prevent
* parallel buffered read to expose the stale data before DIO complete
* the data IO.
*
* As to previously fallocated extents, ext4 get_block will
* just simply mark the buffer mapped but still keep the
* extents unwritten.
* As to previously fallocated extents, ext4 get_block will just simply
* mark the buffer mapped but still keep the extents unwritten.
*
* For non AIO case, we will convert those unwritten extents
* to written after return back from blockdev_direct_IO.
* For non AIO case, we will convert those unwritten extents to written
* after return back from blockdev_direct_IO. That way we save us from
* allocating io_end structure and also the overhead of offloading
* the extent convertion to a workqueue.
*
* For async DIO, the conversion needs to be deferred when the
* IO is completed. The ext4 end_io callback function will be
@@ -3332,30 +3374,13 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
* case, we allocate an io_end structure to hook to the iocb.
*/
iocb->private = NULL;
if (overwrite) {
if (overwrite)
get_block_func = ext4_dio_get_block_overwrite;
else if (is_sync_kiocb(iocb)) {
get_block_func = ext4_dio_get_block_unwritten_sync;
dio_flags = DIO_LOCKING;
} else {
ext4_inode_aio_set(inode, NULL);
if (!is_sync_kiocb(iocb)) {
io_end = ext4_init_io_end(inode, GFP_NOFS);
if (!io_end) {
ret = -ENOMEM;
goto retake_lock;
}
/*
* Grab reference for DIO. Will be dropped in
* ext4_end_io_dio()
*/
iocb->private = ext4_get_io_end(io_end);
/*
* we save the io structure for current async direct
* IO, so that later ext4_map_blocks() could flag the
* io structure whether there is a unwritten extents
* needs to be converted when IO is completed.
*/
ext4_inode_aio_set(inode, io_end);
}
get_block_func = ext4_dio_get_block_unwritten;
get_block_func = ext4_dio_get_block_unwritten_async;
dio_flags = DIO_LOCKING;
}
#ifdef CONFIG_EXT4_FS_ENCRYPTION
@@ -3370,27 +3395,6 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
get_block_func,
ext4_end_io_dio, NULL, dio_flags);
/*
* Put our reference to io_end. This can free the io_end structure e.g.
* in sync IO case or in case of error. It can even perform extent
* conversion if all bios we submitted finished before we got here.
* Note that in that case iocb->private can be already set to NULL
* here.
*/
if (io_end) {
ext4_inode_aio_set(inode, NULL);
ext4_put_io_end(io_end);
/*
* When no IO was submitted ext4_end_io_dio() was not
* called so we have to put iocb's reference.
*/
if (ret <= 0 && ret != -EIOCBQUEUED && iocb->private) {
WARN_ON(iocb->private != io_end);
WARN_ON(io_end->flag & EXT4_IO_END_UNWRITTEN);
ext4_put_io_end(io_end);
iocb->private = NULL;
}
}
if (ret > 0 && !overwrite && ext4_test_inode_state(inode,
EXT4_STATE_DIO_UNWRITTEN)) {
int err;
@@ -3405,7 +3409,6 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
ext4_clear_inode_state(inode, EXT4_STATE_DIO_UNWRITTEN);
}
retake_lock:
if (iov_iter_rw(iter) == WRITE)
inode_dio_end(inode);
/* take i_mutex locking again if we do a ovewrite dio */