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

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(a)virtuozzo.com&gt; Signed-off-by: Daniel P. Berrange <berrange(a)redhat.com&gt; --- src/util/iohelper.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)