[libvirt] [PATCH] iohelper: avoid calling read() with misaligned buffers for O_DIRECT

Eric Blake eblake at redhat.com
Wed Sep 20 18:12:11 UTC 2017


On 09/20/2017 10:31 AM, Daniel P. Berrange wrote:
> The iohelper currently calls saferead() to get data from the
> underlying file. This has a problem with O_DIRECT when hitting
> end-of-file. saferead() is asked to read 1MB, but the first
> read() it does may return only a few KB, so it'll try another
> read() to fill the remaining buffer. Unfortunately the buffer
> pointer passed into this 2nd read() is likely not aligned
> to the extent that O_DIRECT requires, so rather than seeing
> '0' for end-of-file, we'll get -1 + EINVAL due to misaligned
> buffer.
> 
> The way the iohelper is currently written, it already handles
> getting short reads, so there is actually no need to use
> saferead() at all. We can simply call read() directly. The
> benefit of this is that we can now write() the data immediately
> so when we go into the subsequent reads() we'll always have a
> correctly aligned buffer.
> 
> Technically the file position ought to be aligned for O_DIRECT
> too, but this does not appear to matter when at end-of-file.
> 
> Tested-by: Nikolay Shirokovskiy <nshirokovskiy at virtuozzo.com>
> Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
> ---
>  src/util/iohelper.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake at redhat.com>


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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