On Tue, Sep 01, 2015 at 01:08:04PM +0300, Kirill A. Shutemov wrote: > On Tue, Sep 01, 2015 at 09:38:03AM +1000, Dave Chinner wrote: > > On Mon, Aug 31, 2015 at 12:59:44PM -0600, Ross Zwisler wrote: > > Even for DAX, msync has to call vfs_fsync_range() for the filesystem to commit > > the backing store allocations to stable storage, so there's not > > getting around the fact msync is the wrong place to be flushing > > DAX mappings to persistent storage. > > Why? > IIUC, msync() doesn't have any requirements wrt metadata, right? Of course it does. If the backing store allocation has not been committed, then after a crash there will be a hole in file and so it will read as zeroes regardless of what data was written and flushed. > > I pointed this out almost 6 months ago (i.e. that fsync was broken) > > anf hinted at how to solve it. Fix fsync, and msync gets fixed for > > free: > > > > https://lists.01.org/pipermail/linux-nvdimm/2015-March/000341.html > > > > I've also reported to Willy that DAX write page faults don't work > > correctly, either. xfstests generic/080 exposes this: a read > > from a page followed immediately by a write to that page does not > > result in ->page_mkwrite being called on the write and so > > backing store is not allocated for the page, nor are the timestamps > > for the file updated. This will also result in fsync (and msync) > > not working properly. > > Is that because XFS doesn't provide vm_ops->pfn_mkwrite? I didn't know that had been committed. I don't recall seeing a pull request with that in it, none of the XFS DAX patches conflicted against it and there's been no runtime errors. I'll fix it up. As such, shouldn't there be a check in the VM (in ->mmap callers) that if we have the vma is returned with VM_MIXEDMODE enabled that ->pfn_mkwrite is not NULL? It's now clear to me that any filesystem that sets VM_MIXEDMODE needs to support both page_mkwrite and pfn_mkwrite, and such a check would have caught this immediately... 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>