Re: [PATCH] ext4: Flush any pending end_io requests before O_direct read on dioread_nolock
From: Ted Ts'o <tytso@mit.edu>
Date: 2011-08-19 20:57:26
On Thu, Aug 18, 2011 at 06:28:45PM -0700, Jiaying Zhang wrote:
quoted hunk ↗ jump to hunk
@@ -800,12 +800,17 @@ ssize_t ext4_ind_direct_IO(int rw, struct kiocb *iocb, } retry: - if (rw == READ && ext4_should_dioread_nolock(inode)) + if (rw == READ && ext4_should_dioread_nolock(inode)) { + if (unlikely(!list_empty(&ei->i_completed_io_list))) { + mutex_lock(&inode->i_mutex); + ext4_flush_completed_IO(inode); + mutex_unlock(&inode->i_mutex); + }
Doesn't this largely invalidate the reasons for using dioread_nolock in the first place, which was to avoid taking the i_mutex for performance reasons? If we are willing to solve the problem this way, I wonder if we be better off just simply telling users to disable dioread_nolock if they care about cache consistency between DIO reads and buffered writes? (Yes, I do understand that in the hopefully common case where a user is not trying to do buffered writes and DIO reads at the same time, we'll just take and release the mutex very quickly, but still, it's got to have a performance impact.) I seem to recall a conversation I had with Stephen Tweedie over a decade ago, where he noted that many other Unix systems made absolutely no cache consistency guarantees with respect to DIO and the page cache, but he wanted to set a higher standard for Linux. Which is fair enough, but I wonder if for the case of dioread_nolock, since its raison d'etre is to avoid the i_mutex lock, to simply just say that one of the side effects of dioread_nolock is that (for now) the cache consistency guarantees are repealed if this mount option is chosen. What do folks think? - Ted