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: > > For DAX msync we just need to flush the given range using > > wb_cache_pmem(), which is now a public part of the PMEM API. > > This is wrong, because it still leaves fsync() broken on dax. > > Flushing dirty data to stable storage is the responsibility of the > writeback infrastructure, not the VMA/mm infrasrtucture. Writeback infrastructure is non-existent for DAX. Without struct page we don't have anything to transfer pte_ditry() to. And I'm not sure we need to invent some. For DAX flushing in-place can be cheaper than dirty tracking beyond page tables. > For non-dax configurations, msync defers all that to vfs_fsync_range(), > because it has to be implemented there for fsync() to work. Not necessary. I think fsync() for DAX can be implemented with rmap over all file's VMA and msync() them with commiting metadata afterwards. But we also need to commit to persistent on zap_page_range() to make it work. > 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? > 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? -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
