On Tue, Mar 24, 2015 at 08:14:59AM +0200, Boaz Harrosh wrote: > On 03/24/2015 12:40 AM, Dave Chinner wrote: > > On Mon, Mar 23, 2015 at 02:54:40PM +0200, Boaz Harrosh wrote: > >> From: Boaz Harrosh <boaz@xxxxxxxxxxxxx> > >> > >> When freezing an FS, we must write protect all IS_DAX() > >> inodes that have an mmap mapping on an inode. Otherwise > >> application will be able to modify previously faulted-in > >> file pages. > > > > All you need to do is lock out page faults once the page is clean; > > that's what the sb_start_pagefault() calls are for in the page fault > > path - they catch write faults and block them until the filesystem > > is unfrozen. Hence I'm not sure why this would be necessary if you > > are catching write faults in .pfn_mkwrite.... > > > > Jan pointed it out and he was right I have a test for this. What > happens is that since we had a mapping from before the freeze we will > not have a page-fault. And the buffers will be modified. > > As Jan explained in the cache path we do a writeback which turns > all pages to read-only. But with dax we do not have writeback > so the pages stay read-write mapped. Something needs to loop > through the pages and write-protect them. I chose to unmap > them because it is the much-much smaller code, and I do not care > to optimize the freeze. Then we have wider problem with DAX, then: sync doesn't work properly. i.e. if we still has write mapped pages, then we haven't flushed dirty cache lines on write-mapped files to the persistent domain by the time sync completes. So, this shouldn't be some special case that only the freeze code takes into account - we need to make sure that sync (and therefore freeze) flushes all dirty cache lines and marks all mappings clean.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>