Re: [PATCH v3 2/7] util/oslib-posix: Support MADV_POPULATE_WRITE for os_mem_prealloc()

[Daniel P . Berrangé]

On Mon, Aug 16, 2021 at 11:47:34AM +0200, David Hildenbrand wrote:
> Let's sense support and use it for preallocation. MADV_POPULATE_WRITE
> does not require a SIGBUS handler, doesn't actually touch page content,
> and avoids context switches; it is, therefore, faster and easier to handle
> than our current approach.
> 
> While MADV_POPULATE_WRITE is, in general, faster than manual
> prefaulting, and especially faster with 4k pages, there is still value in
> prefaulting using multiple threads to speed up preallocation.
> 
> More details on MADV_POPULATE_WRITE can be found in the Linux commits
> 4ca9b3859dac ("mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault
> page tables") and eb2faa513c24 ("mm/madvise: report SIGBUS as -EFAULT for
> MADV_POPULATE_(READ|WRITE)"), and in the man page proposal [1].
> 
> This resolves the TODO in do_touch_pages().
> 
> In the future, we might want to look into using fallocate(), eventually
> combined with MADV_POPULATE_READ, when dealing with shared file/fd
> mappings and not caring about memory bindings.
> 
> [1] https://lkml.kernel.org/r/20210816081922.5155-1-david@redhat.com
> 
> Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/qemu/osdep.h |  7 ++++
>  util/oslib-posix.c   | 83 +++++++++++++++++++++++++++++++++-----------
>  2 files changed, 69 insertions(+), 21 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


Regards,
Daniel
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