Re: [PATCH 3/3] filemap: don't call generic_write_sync for -EIOCBQUEUED
From: Jan Kara <jack@suse.cz>
Date: 2012-02-02 17:52:19
Also in:
linux-fsdevel, linux-xfs
Hello, On Fri 27-01-12 16:15:49, Jeff Moyer wrote:
As it stands, generic_file_aio_write will call into generic_write_sync when -EIOCBQUEUED is returned from __generic_file_aio_write. EIOCBQUEUED indicates that an I/O was submitted but NOT completed. Thus, we will flush the disk cache, potentially before the write(s) even make it to the disk!
Yeah. It seems to be a problem introduced by Tejun's rewrite of barrier code, right? Before that we'd drain the IO queue when cache flush is issued and thus effectively wait for IO completion...
Up until now, this has been the best we could do, as file systems didn't bother to flush the disk cache after an O_SYNC AIO+DIO write. After applying the prior two patches to xfs and ext4, at least the major two file systems do the right thing. So, let's go ahead and fix this backwards logic.
But doesn't this break filesystems which you didn't fix explicitely even more than they were? You are right they might have sent cache flush too early but they'd at least propely force all metadata modifications (e.g. from allocation) to disk. But after this patch O_SYNC will have simply no effect for these filesystems. Also I was thinking whether we couldn't implement the fix in VFS. Basically it would be the same like the fix for ext4. Like having a per-sb workqueue and queue work calling generic_write_sync() from end_io handler when the file is O_SYNC? That would solve the issue for all filesystems... Honza
quoted hunk ↗ jump to hunk
Signed-off-by: Jeff Moyer <redacted> --- mm/filemap.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)diff --git a/mm/filemap.c b/mm/filemap.c index c4ee2e9..004442f 100644 --- a/mm/filemap.c +++ b/mm/filemap.c@@ -2634,7 +2634,7 @@ ssize_t generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov, ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos); mutex_unlock(&inode->i_mutex); - if (ret > 0 || ret == -EIOCBQUEUED) { + if (ret > 0) { ssize_t err; err = generic_write_sync(file, pos, ret);-- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
-- Jan Kara [off-list ref] SUSE Labs, CR