Re: [PATCH v4 1/3] vhost: Rework memslot filtering and fix "used_memslot" tracking

[Peter Xu <peterx@redhat.com>]

On Tue, May 23, 2023 at 08:59:13PM +0200, David Hildenbrand wrote:
> Having multiple vhost devices, some filtering out fd-less memslots and
> some not, can mess up the "used_memslot" accounting. Consequently our
> "free memslot" checks become unreliable and we might run out of free
> memslots at runtime later.
> 
> An example sequence which can trigger a potential issue that involves
> different vhost backends (vhost-kernel and vhost-user) and hotplugged
> memory devices can be found at [1].
> 
> Let's make the filtering mechanism less generic and distinguish between
> backends that support private memslots (without a fd) and ones that only
> support shared memslots (with a fd). Track the used_memslots for both
> cases separately and use the corresponding value when required.
> 
> Note: Most probably we should filter out MAP_PRIVATE fd-based RAM regions
> (for example, via memory-backend-memfd,...,shared=off or as default with
>  memory-backend-file) as well. When not using MAP_SHARED, it might not work
> as expected. Add a TODO for now.
> 
> [1] https://lkml.kernel.org/r/fad9136f-08d3-3fd9-71a1-502069c000cf@redhat.com
> 
> Fixes: 988a27754bbb ("vhost: allow backends to filter memory sections")
> Cc: Tiwei Bie <tiwei.bie@intel.com>
> Acked-by: Igor Mammedov <imammedo@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu