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vhost: Rework memslot filtering and fix "used_memslot" tracking
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 Message-ID: <20230926185738.277351-2-david@redhat.com> Fixes: 988a277 ("vhost: allow backends to filter memory sections") Cc: Tiwei Bie <tiwei.bie@intel.com> Acked-by: Igor Mammedov <imammedo@redhat.com> Reviewed-by: Peter Xu <peterx@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David Hildenbrand <david@redhat.com>
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