Greetings, I've run into a case where the fsync() system call seems to have returned before all file data was actually on disk. (A SLES11SP1 system crash occurred shortly after an fsync which had returned zero. After restarting the machine, the last I/O before the fsync is not in the file.) In attempting to find the problem, I've come across code I don't understand, and am hoping someone can enlighten me as to how things are supposed to work. Routine xfs_vm_writepage has various situations under which it will decide it can't currently initiate writeback on a page, and in that case calls redirty_page_for_writepage, unlocks the page, and returns zero. That seems to me to be incompatible with fsync(), so I'm obviously missing some key piece of logic. The calling sequence of routines involved in fsync is: do_fsync->vfs_fsync->vfs_fsync_range-> filemap_write_and_wait_range-> __filemap_fdatawrite_range-> do_writepages->generic_writepages-> write_cache_pages Routine write_cache_pages walks the radix tree and calls clear_page_dirty_for_io and then __writepage on each dirty page to initiate writeback. __writepage calls xfs_vm_writepage. That routine is occasionally unable to immediately start writeback of the page, and so it calls redirty_page_for_writepage without setting the writeback flag. When write_cache_pages resumes after the __writepage call, it continues walking the radix tree starting additional writebacks on dirty pages, but nothing I can see will ever come back and try again to start a writeback on the page that xfs_vm_writepage couldn't writeback. Eventually control bubbles back up to filemap_write_and_wait_range() where wait_on_page_writeback_range is called, but that routine only waits for writebacks to complete, it doesn't do anything about dirty pages. So it appears to me that the dirty page will be left dirty indefinitely even though the wbc contained WB_SYNC_ALL. I'd like to believe that I am missing something, and that the code is correct, but I do have a crash dump where I can see dirty pages in files that were recently fsync'd. And I can't believe the problem is something inside XFS, because I see other filesystems also call redirty_page_for_writepage, so I think the same problem could occur with them. Could someone please describe to me how fsync is supposed to work in combination with xfs_vm_writepage? Thanks in advance, Regards, Kevan _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
