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dbe1a27745
Currently, virtio-balloon uses madvise() with MADV_DONTNEED to actually discard RAM pages inserted into the balloon. This is basically a Linux only interface (MADV_DONTNEED exists on some other platforms, but doesn't always have the same semantics). It also doesn't work on hugepages and has some other limitations. It turns out that postcopy also needs to discard chunks of memory, and uses a better interface for it: ram_block_discard_range(). It doesn't cover every case, but it covers more than going direct to madvise() and this gives us a single place to update for more possibilities in future. There are some subtleties here to maintain the current balloon behaviour: * For now, we just ignore requests to balloon in a hugepage backed region. That matches current behaviour, because MADV_DONTNEED on a hugepage would simply fail, and we ignore the error. * If host page size is > BALLOON_PAGE_SIZE we can frequently call this on non-host-page-aligned addresses. These would also fail in madvise(), which we then ignored. ram_block_discard_range() error_report()s calls on unaligned addresses, so we explicitly check that case to avoid spamming the logs. * We now call ram_block_discard_range() with the *host* page size, whereas we previously called madvise() with BALLOON_PAGE_SIZE. Surprisingly, this also matches existing behaviour. Although the kernel fails madvise on unaligned addresses, it will round unaligned sizes *up* to the host page size. Yes, this means that if BALLOON_PAGE_SIZE < guest page size we can incorrectly discard more memory than the guest asked us to. I'm planning to address that soon. Errors other than the ones discussed above, will now be reported by ram_block_discard_range(), rather than silently ignored, which means we have a much better chance of seeing when something is going wrong. Signed-off-by: David Gibson <david@gibson.dropbear.id.au> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Message-Id: <20190214043916.22128-5-david@gibson.dropbear.id.au> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> |
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| Makefile.objs | ||