From: Jeff Layton Subject: [PATCH v4 25/27] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors Date: Tue, 9 May 2017 11:49:28 -0400 Message-ID: <20170509154930.29524-26-jlayton@redhat.com> References: <20170509154930.29524-1-jlayton@redhat.com> Cc: axboe@kernel.dk, tytso@mit.edu, mawilcox@microsoft.com, neilb@suse.de, corbet@lwn.net, josef@toxicpanda.com, dhowells@redhat.com, clm@fb.com, bo.li.liu@oracle.com, viro@zeniv.linux.org.uk, jack@suse.com, akpm@linux-foundation.org, ross.zwisler@linux.intel.com, hubcap@omnibond.com To: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-cifs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, jfs-discussion@lists.sourceforge.net, linux-xfs@vger.kernel.org, cluster-devel@redhat.com, linux-f2fs-devel@lists.sourceforge.net, v9fs-developer@lists.sourceforge.net, linux-nilfs@vger.kernel.org, linux-block@vger.kernel.org Return-path: In-Reply-To: <20170509154930.29524-1-jlayton@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: cluster-devel-bounces@redhat.com Errors-To: cluster-devel-bounces@redhat.com List-Id: linux-ext4.vger.kernel.org I waxed a little loquacious here, but I figured that more detail was better, and writeback error handling is so hard to get right. Cc: Jan Kara Signed-off-by: Jeff Layton --- Documentation/filesystems/vfs.txt | 54 ++++++++++++++++++++++++++++++++------- 1 file changed, 45 insertions(+), 9 deletions(-) diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index f201a77873f7..382190a872e5 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -576,12 +576,46 @@ should clear PG_Dirty and set PG_Writeback. It can be actually written at any point after PG_Dirty is clear. Once it is known to be safe, PG_Writeback is cleared. -If there is an error during writeback, then the address_space should be -marked with an error (typically using mapping_set_error), in order to -ensure that the error can later be reported to the application when an -fsync is issued. - -Writeback makes use of a writeback_control structure... +Writeback makes use of a writeback_control structure to direct the +operations. This gives the the writepage and writepages operations some +information about the nature of and reason for the writeback request, +and the constraints under which it is being done. It is also used to +return information back to the caller about the result of a writepage or +writepages request. + +Handling errors during writeback +-------------------------------- +Most applications that utilize the pagecache will periodically call +fsync to ensure that data written has made it to the backing store. +When there is an error during writeback, that error should be reported +when fsync is called. After an error has been reported to fsync, +subsequent fsync calls on the same file descriptor should return 0, +unless further writeback errors have occurred since the previous fsync. + +Ideally, the kernel would report it only on file descriptions on which +writes were done that subsequently failed to be written back. The +generic pagecache infrastructure does not track the file descriptions +that have dirtied each individual page however, so determining which +file descriptors should get back an error is not possible. + +Instead, the generic writeback error tracking infrastructure in the +kernel settles for reporting errors to fsync on all file descriptions +that were open at the time that the error occurred. In a situation with +multiple writers, all of them will get back an error on a subsequent fsync, +even if all of the writes done through that particular file descriptor +succeeded (or even if there were no writes on that file descriptor at all). + +Filesystems that wish to use this infrastructure should call +mapping_set_error to record the error in the address_space when it +occurs. The generic vfs code will then handle reporting the error when +fsync is called, even if the fsync file operation returned 0. + +Filesystems are free to track errors internally if they choose (i.e. if +they do keep track of how the pages were dirtied), but they should aim +to provide the same (or better) error reporting semantics for when there +are multiple writers. Those filesystems should avoid calling +mapping_set_error in order to ensure that errors stored in the mapping +aren't improperly reported by the generic filesystem code. struct address_space_operations ------------------------------- @@ -810,7 +844,8 @@ struct address_space_operations { The File Object =============== -A file object represents a file opened by a process. +A file object represents a file opened by a process. This is also known +as an "open file description" in POSIX parlance. struct file_operations @@ -893,9 +928,10 @@ otherwise noted. release: called when the last reference to an open file is closed - fsync: called by the fsync(2) system call. Errors that were previously + fsync: called by the fsync(2) system call. Errors that were previously recorded using mapping_set_error will automatically be returned to - the application and the file's error sequence advanced. + the application and the struct file's error sequence advanced. + See the section above on handling writeback errors. fasync: called by the fcntl(2) system call when asynchronous (non-blocking) mode is enabled for a file -- 2.9.3