[Libguestfs] ZFS-on-NBD

[Nikolaus Rath]

On Sep 14 2022, Shaun McDowell <shaunjmcdowell at gmail.com> wrote:
> It has been a few years since I've worked on it but there were a number of
> gotchyas we had to overcome when running block devices in userspace and
> mounted on the same system. The largest of which is that the userland
> process needs to be extremely careful with memory allocations (ideally all
> memory will be allocated at startup of the userland driver and memlocked)
> because you can quite easily cause deadlock on the system if your userland
> block device driver needs to allocate memory at any point during an
> operation and the kernel decides it first needs to reclaim memory and
> chooses to clean out dirty filesystem buffer pages to satisfy the malloc.
> This causes the kernel to again try to re-enter the filesystem and block
> device kernel drivers to flush those dirty pages and that causes deadlocks
> in the kernel as locks were already being held by the previous
> operation.

Is this not solved by PR_SET_IO_FLUSHER?

Best,
-Nikolaus

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