Re: [PATCH] dax, pmem: add support for msync

From: Kirill A. Shutemov
Date: Tue Sep 01 2015 - 06:08:14 EST

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

> 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.

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:
> 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
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