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SubjectRe: [PATCH v4 5/5] virtiofs: propagate sync() to file server
Hi Greg,

Sorry for the late reply, I have some questions about this change...

On Fri, May 21, 2021 at 9:12 AM Greg Kurz <groug@kaod.org> wrote:
>
> Even if POSIX doesn't mandate it, linux users legitimately expect
> sync() to flush all data and metadata to physical storage when it
> is located on the same system. This isn't happening with virtiofs
> though : sync() inside the guest returns right away even though
> data still needs to be flushed from the host page cache.
>
> This is easily demonstrated by doing the following in the guest:
>
> $ dd if=/dev/zero of=/mnt/foo bs=1M count=5K ; strace -T -e sync sync
> 5120+0 records in
> 5120+0 records out
> 5368709120 bytes (5.4 GB, 5.0 GiB) copied, 5.22224 s, 1.0 GB/s
> sync() = 0 <0.024068>
> +++ exited with 0 +++
>
> and start the following in the host when the 'dd' command completes
> in the guest:
>
> $ strace -T -e fsync /usr/bin/sync virtiofs/foo
> fsync(3) = 0 <10.371640>
> +++ exited with 0 +++
>
> There are no good reasons not to honor the expected behavior of
> sync() actually : it gives an unrealistic impression that virtiofs
> is super fast and that data has safely landed on HW, which isn't
> the case obviously.
>
> Implement a ->sync_fs() superblock operation that sends a new
> FUSE_SYNCFS request type for this purpose. Provision a 64-bit
> placeholder for possible future extensions. Since the file
> server cannot handle the wait == 0 case, we skip it to avoid a
> gratuitous roundtrip. Note that this is per-superblock : a
> FUSE_SYNCFS is send for the root mount and for each submount.
>
> Like with FUSE_FSYNC and FUSE_FSYNCDIR, lack of support for
> FUSE_SYNCFS in the file server is treated as permanent success.
> This ensures compatibility with older file servers : the client
> will get the current behavior of sync() not being propagated to
> the file server.

I wonder - even if the server does not support SYNCFS or if the kernel
does not trust the server with SYNCFS, fuse_sync_fs() can wait
until all pending requests up to this call have been completed, either
before or after submitting the SYNCFS request. No?

Does virtiofsd track all requests prior to SYNCFS request to make
sure that they were executed on the host filesystem before calling
syncfs() on the host filesystem?

I am not familiar enough with FUSE internals so there may already
be a mechanism to track/wait for all pending requests?

>
> Note that such an operation allows the file server to DoS sync().
> Since a typical FUSE file server is an untrusted piece of software
> running in userspace, this is disabled by default. Only enable it
> with virtiofs for now since virtiofsd is supposedly trusted by the
> guest kernel.

Isn't there already a similar risk of DoS to sync() from the ability of any
untrusted (or malfunctioning) server to block writes?

Thanks,
Amir.

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