Stefan Hajnoczi
2021-Dec-15 10:30 UTC
[PATCH 4/5] dax: remove the copy_from_iter and copy_to_iter methods
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 at 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 at 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. Stefan -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 488 bytes Desc: not available URL: <http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20211215/da10391c/attachment-0001.sig>
Vivek Goyal
2021-Dec-15 15:43 UTC
[PATCH 4/5] dax: remove the copy_from_iter and copy_to_iter methods
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 at 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 at 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