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

[Dave Chinner <david@fromorbit.com>]

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@fromorbit.com

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