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Re: [Qemu-devel] [PATCH v4 0/6] nvdimm: support MAP_SYNC for memory-back

From: Dan Williams
Subject: Re: [Qemu-devel] [PATCH v4 0/6] nvdimm: support MAP_SYNC for memory-backend-file
Date: Wed, 31 Jan 2018 16:08:30 -0800

On Wed, Jan 31, 2018 at 4:02 PM, Haozhong Zhang
<address@hidden> wrote:
> On 01/31/18 14:25 -0800, Dan Williams wrote:
>> On Tue, Jan 30, 2018 at 10:02 PM, Haozhong Zhang
>> <address@hidden> wrote:
>> > Linux 4.15 introduces a new mmap flag MAP_SYNC, which can be used to
>> > guarantee the write persistence to mmap'ed files supporting DAX (e.g.,
>> > files on ext4/xfs file system mounted with '-o dax').
>> Wait, MAP_SYNC does not guarantee persistence. It makes sure that the
>> metadata is in sync after a fault. However, that does not make
>> filesystem-DAX safe for use with QEMU, because we still need to
>> coordinate DMA with fileystem operations. There is no way to do that
>> coordination from within a guest. QEMU needs to use device-dax if the
>> guest might ever perform DMA to a virtual-pmem range. See this patch
>> set for more details on the DAX vs DMA problem [1]. I think we need to
>> enforce this in the host kernel. I.e. do not allow file backed DAX
>> pages to be mapped in EPT entries unless / until we have a solution to
>> the DMA synchronization problem. Apologies for not noticing this
>> earlier.
> QEMU does not truncate or punch holes of the file once it has been
> mmap()'ed. Does the problem [1] still exist in such case?

Something else on the system might. The only agent that could enforce
protection is the kernel, and the kernel will likely just disallow
passing addresses from filesystem-dax vmas through to a guest
altogether. I think there's even a problem in the non-DAX case unless
KVM is pinning pages while they are handed out to a guest. The problem
is that we don't have a page cache page to pin in the DAX case.

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