Hello Jan,
Thanks for a thorough review.
On 11/20/19 7:25 PM, Jan Kara wrote:
On Wed 20-11-19 10:30:23, Ritesh Harjani wrote:
Earlier there was no shared lock in DIO read path.
But this patch (16c54688592ce: ext4: Allow parallel DIO reads)
simplified some of the locking mechanism while still allowing
for parallel DIO reads by adding shared lock in inode DIO
read path.
But this created problem with mixed read/write workload.
It is due to the fact that in DIO path, we first start with
exclusive lock and only when we determine that it is a ovewrite
IO, we downgrade the lock. This causes the problem, since
with above patch we have shared locking in DIO reads.
So, this patch tries to fix this issue by starting with
shared lock and then switching to exclusive lock only
when required based on ext4_dio_write_checks().
Other than that, it also simplifies below cases:-
1. Simplified ext4_unaligned_aio API to ext4_unaligned_io.
Previous API was abused in the sense that it was not really checking
for AIO anywhere also it used to check for extending writes.
So this API was renamed and simplified to ext4_unaligned_io()
which actully only checks if the IO is really unaligned.
Now, in case of unaligned direct IO, iomap_dio_rw needs to do
zeroing of partial block and that will require serialization
against other direct IOs in the same block. So we take a excl iolock
for any unaligned DIO.
In case of AIO we also need to wait for any outstanding IOs to
complete so that conversion from unwritten to written is completed
before anyone try to map the overlapping block. Hence we take
excl iolock and also wait for inode_dio_wait() for unaligned DIO case.
Please note since we are anyway taking an exclusive lock in unaligned IO,
inode_dio_wait() becomes a no-op in case of non-AIO DIO.
2. Added ext4_extending_io(). This checks if the IO is extending the file.
3. Added ext4_dio_write_checks().
In this we start with shared iolock and only switch to exclusive iolock
if required. So in most cases with aligned, non-extening, dioread_nolock
overwrites tries to write with a shared locking.
If not, then we restart the operation in ext4_dio_write_checks(),
after acquiring excl iolock.
Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
---
fs/ext4/file.c | 191 ++++++++++++++++++++++++++++++++++++-------------
1 file changed, 142 insertions(+), 49 deletions(-)
Thanks for the patch! Some comments below...
@@ -365,15 +383,110 @@ static const struct iomap_dio_ops ext4_dio_write_ops = {
.end_io = ext4_dio_write_end_io,
};
+/*
+ * The intention here is to start with shared lock acquired then see if any
+ * condition requires an exclusive inode lock. If yes, then we restart the
+ * whole operation by releasing the shared lock and acquiring exclusive lock.
+ *
+ * - For unaligned_io we never take shared lock as it may cause data corruption
+ * when two unaligned IO tries to modify the same block e.g. while zeroing.
+ *
+ * - For extending writes case we don't take the shared lock, since it requires
+ * updating inode i_disksize and/or orphan handling with exclusive lock.
+ *
+ * - shared locking will only be true mostly in case of overwrites with
+ * dioread_nolock mode. Otherwise we will switch to excl. iolock mode.
+ */
+static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
+ unsigned int *iolock, bool *unaligned_io,
+ bool *extend)
+{
+ struct file *file = iocb->ki_filp;
+ struct inode *inode = file_inode(file);
+ loff_t offset = iocb->ki_pos;
+ loff_t final_size;
+ size_t count;
+ ssize_t ret;
+
+restart:
+ /* Fallback to buffered I/O if the inode does not support direct I/O. */
+ if (!ext4_dio_supported(inode)) {
+ ext4_iunlock(inode, *iolock);
+ return ext4_buffered_write_iter(iocb, from);
+ }
I don't think it is good to hide things like this fallback to buffered IO
in ext4_dio_write_checks(). Similarly with 'unaligned_io' and 'extend'
Yes, make sense. Yup will move above block of code in
ext4_dio_write_iter() before calling for ext4_dio_write_checks().
variables below. So I'd rather leave this in ext4_dio_write_iter() and just
move file_modified() from ext4_write_checks() since that seems to be the
Yes, in this patch itself that has been done. We removed file_modified()
from ext4_write_checks() & renamed it to ext4_generic_write_checks().
only thing that cannot be always done with shared i_rwsem, can it?
AFAIU, that's right. Exclusive lock must be needed to change the
security info for inode.
+
+ ret = ext4_generic_write_checks(iocb, from);
+ if (ret <= 0) {
+ ext4_iunlock(inode, *iolock);
+ return ret;
+ }
+
+ /* Recalculate since offset & count may change above. */
+ offset = iocb->ki_pos;
+ count = iov_iter_count(from);
+ final_size = offset + count;
+
+ if (ext4_unaligned_io(inode, from, offset))
+ *unaligned_io = true;
No need to recheck alignment here. That cannot change over time regardless
of locks we hold...
hmm. I think I got confused by function iov_iter_truncate() called
in ext4_generic_write_checks().
But looking at it again, I agree that alignment won't change here.
will remove the alignment check from here.
+
+ if (ext4_extending_io(inode, offset, count))
+ *extend = true;
+ /*
+ * Determine whether the IO operation will overwrite allocated
+ * and initialized blocks. If so, check to see whether it is
+ * possible to take the dioread_nolock path.
+ *
+ * We need exclusive i_rwsem for changing security info
+ * in file_modified().
+ */
+ if (*iolock == EXT4_IOLOCK_SHARED &&
+ (!IS_NOSEC(inode) || *unaligned_io || *extend ||
+ !ext4_should_dioread_nolock(inode) ||
+ !ext4_overwrite_io(inode, offset, count))) {
+ ext4_iunlock(inode, *iolock);
+ *iolock = EXT4_IOLOCK_EXCL;
+ ext4_ilock(inode, *iolock);
+ goto restart;
+ }
+
+ ret = file_modified(file);
+ if (ret < 0) {
+ ext4_iunlock(inode, *iolock);
+ return ret;
+ }
+
+ return count;
+}
+
static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
ssize_t ret;
- size_t count;
- loff_t offset;
handle_t *handle;
struct inode *inode = file_inode(iocb->ki_filp);
- bool extend = false, overwrite = false, unaligned_aio = false;
- unsigned int iolock = EXT4_IOLOCK_EXCL;
+ loff_t offset = iocb->ki_pos;
+ size_t count = iov_iter_count(from);
+ bool extend = false, unaligned_io = false;
+ unsigned int iolock = EXT4_IOLOCK_SHARED;
+
+ /*
+ * We initially start with shared inode lock
+ * unless it is unaligned IO which needs
+ * exclusive lock anyways.
+ */
+ if (ext4_unaligned_io(inode, from, offset)) {
+ unaligned_io = true;
+ iolock = EXT4_IOLOCK_EXCL;
+ }
+ /*
+ * Extending writes need exclusive lock.
+ */
+ if (ext4_extending_io(inode, offset, count)) {
+ extend = true;
+ iolock = EXT4_IOLOCK_EXCL;
+ }
You cannot read EXT4_I(inode)->i_disksize without some lock (either
inode->i_rwsem or EXT4_I(inode)->i_data_sem). So I'd just do here a quick
check with i_size here (probably don't set extend, but just make note to
start with exclusive i_rwsem) and later when we hold i_rwsem, we can do a
reliable check.
hmm. my bad. I think I later moved this block of code from
ext4_dio_write_checks() down here.
Thanks for correcting it.
Will only check against 'i_size' here then along with a
comment that a more reliable check with
shared lock has been done in ext4_dio_write_checks()
via ext4_extending_io().
+ if (iolock == EXT4_IOLOCK_SHARED && !ext4_should_dioread_nolock(inode))
+ iolock = EXT4_IOLOCK_EXCL;
if (iocb->ki_flags & IOCB_NOWAIT) {
if (!ext4_ilock_nowait(inode, iolock))
@@ -382,47 +495,28 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
ext4_ilock(inode, iolock);
}
- if (!ext4_dio_supported(inode)) {
- ext4_iunlock(inode, iolock);
- /*
- * Fallback to buffered I/O if the inode does not support
- * direct I/O.
- */
- return ext4_buffered_write_iter(iocb, from);
- }
-
- ret = ext4_write_checks(iocb, from);
- if (ret <= 0) {
- ext4_iunlock(inode, iolock);
+ ret = ext4_dio_write_checks(iocb, from, &iolock, &unaligned_io,
+ &extend);
+ if (ret <= 0)
return ret;
- }
- /*
- * Unaligned asynchronous direct I/O must be serialized among each
- * other as the zeroing of partial blocks of two competing unaligned
- * asynchronous direct I/O writes can result in data corruption.
- */
offset = iocb->ki_pos;
count = iov_iter_count(from);
- if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
- !is_sync_kiocb(iocb) && ext4_unaligned_aio(inode, from, offset)) {
- unaligned_aio = true;
- inode_dio_wait(inode);
- }
/*
- * Determine whether the I/O will overwrite allocated and initialized
- * blocks. If so, check to see whether it is possible to take the
- * dioread_nolock path.
+ * Unaligned direct IO must be serialized among each other as zeroing
+ * of partial blocks of two competing unaligned IOs can result in data
+ * corruption.
+ *
+ * So we make sure we don't allow any unaligned IO in flight.
+ * For IOs where we need not wait (like unaligned non-AIO DIO),
+ * below inode_dio_wait() may anyway become a no-op, since we start
+ * with exclusive lock.
*/
- if (!unaligned_aio && ext4_overwrite_io(inode, offset, count) &&
- ext4_should_dioread_nolock(inode)) {
- overwrite = true;
- ext4_ilock_demote(inode, iolock);
- iolock = EXT4_IOLOCK_SHARED;
- }
+ if (unaligned_io)
+ inode_dio_wait(inode);
- if (offset + count > EXT4_I(inode)->i_disksize) {
+ if (extend) {
handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
if (IS_ERR(handle)) {
ret = PTR_ERR(handle);
@@ -435,12 +529,11 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
goto out;
}
- extend = true;
ext4_journal_stop(handle);
}
ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, &ext4_dio_write_ops,
- is_sync_kiocb(iocb) || unaligned_aio || extend);
+ is_sync_kiocb(iocb) || unaligned_io || extend);
if (extend)
ret = ext4_handle_inode_extension(inode, offset, ret, count);
Honza
