Re: [PATCH 4/5] dax: remove the copy_from_iter and copy_to_iter methods

Vivek Goyal
2021-12-15T16:01:19Z

On Wed, Dec 15, 2021 at 10:30:50AM +0000, Stefan Hajnoczi wrote:
> On Tue, Dec 14, 2021 at 03:32:43PM -0500, Vivek Goyal wrote:
> > On Tue, Dec 14, 2021 at 08:41:30AM -0800, Dan Williams wrote:
> > > On Tue, Dec 14, 2021 at 6:23 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > >
> > > > On Mon, Dec 13, 2021 at 09:23:18AM +0100, Christoph Hellwig wrote:
> > > > > On Sun, Dec 12, 2021 at 06:44:26AM -0800, Dan Williams wrote:
> > > > > > On Fri, Dec 10, 2021 at 6:17 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > > > > > Going forward, I am wondering should virtiofs use flushcache version as
> > > > > > > well. What if host filesystem is using DAX and mapping persistent memory
> > > > > > > pfn directly into qemu address space. I have never tested that.
> > > > > > >
> > > > > > > Right now we are relying on applications to do fsync/msync on virtiofs
> > > > > > > for data persistence.
> > > > > >
> > > > > > This sounds like it would need coordination with a paravirtualized
> > > > > > driver that can indicate whether the host side is pmem or not, like
> > > > > > the virtio_pmem driver. However, if the guest sends any fsync/msync
> > > > > > you would still need to go explicitly cache flush any dirty page
> > > > > > because you can't necessarily trust that the guest did that already.
> > > > >
> > > > > Do we? The application can't really know what backend it is on, so
> > > > > it sounds like the current virtiofs implementation doesn't really, does it?
> > > >
> > > > Agreed that application does not know what backend it is on. So virtiofs
> > > > just offers regular posix API where applications have to do fsync/msync
> > > > for data persistence. No support for mmap(MAP_SYNC). We don't offer persistent
> > > > memory programming model on virtiofs. That's not the expectation. DAX
> > > > is used only to bypass guest page cache.
> > > >
> > > > With this assumption, I think we might not have to use flushcache version
> > > > at all even if shared filesystem is on persistent memory on host.
> > > >
> > > > - We mmap() host files into qemu address space. So any dax store in virtiofs
> > > > should make corresponding pages dirty in page cache on host and when
> > > > and fsync()/msync() comes later, it should flush all the data to PMEM.
> > > >
> > > > - In case of file extending writes, virtiofs falls back to regular
> > > > FUSE_WRITE path (and not use DAX), and in that case host pmem driver
> > > > should make sure writes are flushed to pmem immediately.
> > > >
> > > > Are there any other path I am missing. If not, looks like we might not
> > > > have to use flushcache version in virtiofs at all as long as we are not
> > > > offering guest applications user space flushes and MAP_SYNC support.
> > > >
> > > > We still might have to use machine check safe variant though as loads
> > > > might generate synchronous machine check. What's not clear to me is
> > > > that if this MC safe variant should be used only in case of PMEM or
> > > > should it be used in case of non-PMEM as well.
> > >
> > > It should be used on any memory address that can throw exception on
> > > load, which is any physical address, in paths that can tolerate
> > > memcpy() returning an error code, most I/O paths, and can tolerate
> > > slower copy performance on older platforms that do not support MC
> > > recovery with fast string operations, to date that's only PMEM users.
> >
> > Ok, So basically latest cpus can do fast string operations with MC
> > recovery so that using MC safe variant is not a problem.
> >
> > Then there is range of cpus which can do MC recovery but do slower
> > versions of memcpy and that's where the issue is.
> >
> > So if we knew that virtiofs dax window is backed by a pmem device
> > then we should always use MC safe variant. Even if it means paying
> > the price of slow version for the sake of correctness.
> >
> > But if we are not using pmem on host, then there is no point in
> > using MC safe variant.
> >
> > IOW.
> >
> > if (virtiofs_backed_by_pmem) {
> > use_mc_safe_version
> > else
> > use_non_mc_safe_version
> > }
> >
> > Now question is, how do we know if virtiofs dax window is backed by
> > a pmem or not. I checked virtio_pmem driver and that does not seem
> > to communicate anything like that. It just communicates start of the
> > range and size of range, nothing else.
> >
> > I don't have full handle on stack of modules of virtio_pmem, but my guess
> > is it probably is using MC safe version always (because it does not
> > know anthing about the backing storage).
> >
> > /me will definitely like to pay penalty of slower memcpy if virtiofs
> > device is not backed by a pmem.
>
> Reads from the page cache handle machine checks (filemap_read() ->
> raw_copy_to_user()). I think virtiofs should therefore always handle
> machine checks when reading from the DAX Window.

IIUC, raw_copy_to_user() does not handle recovery from machine check. For
example, it can call copy_user_enhanced_fast_string() if cpu supports
X86_FEATURE_ERMS. But equivalent machine check safe version is
copy_mc_enhanced_fast_string() instead.

Hence, I don't think reading from page cache is using machine check safe
variants by default. This copy_mc_to_user() path has to be taken
explicitly for machine check safe variants. And currently only pmem driver
seems to take it by calling _copy_mc_to_iter().

Thanks
Vivek