Re: [Qemu-devel] [Qemu-block] [PATCH] block, migration: Use qemu_madvise inplace of madvise

[Pankaj Gupta <pagupta@redhat.com>]

Thanks for your comments. I have below query.
> 
> On Fri 17 Feb 2017 09:06:04 AM CET, Pankaj Gupta wrote:
> >  To maintain consistency at all the places use qemu_madvise wrapper
> >  inplace of madvise call.
> 
> >      if (length > 0) {
> > -        madvise((uint8_t *) t + offset, length, MADV_DONTNEED);
> > +        qemu_madvise((uint8_t *) t + offset, length, QEMU_MADV_DONTNEED);
> 
> This was changed two months ago from qemu_madvise() to madvise(), is
> there any reason why you want to revert that change? Those two calls are
> not equivalent, please see commit 2f2c8d6b371cfc6689affb0b7e for an
> explanation.
> 
> > -    if (madvise(start, length, MADV_DONTNEED)) {
> > +    if (qemu_madvise(start, length, QEMU_MADV_DONTNEED)) {
> >          error_report("%s MADV_DONTNEED: %s", __func__, strerror(errno));

I checked history of only change related to 'postcopy'.

For my linux machine:

./config-host.mak

CONFIG_MADVISE=y
CONFIG_POSIX_MADVISE=y

As both these options are set for Linux, every time we call call 'qemu_madvise' ==>"madvise(addr, len, advice);" will 
be compiled/called. I don't understand why '2f2c8d6b371cfc6689affb0b7e' explicitly changed for :"#ifdef CONFIG_LINUX"
I think its better to write generic function maybe in a wrapper then to conditionally set something at different places.

int qemu_madvise(void *addr, size_t len, int advice)
{
    if (advice == QEMU_MADV_INVALID) {
        errno = EINVAL;
        return -1;
    }
#if defined(CONFIG_MADVISE)
    return madvise(addr, len, advice);
#elif defined(CONFIG_POSIX_MADVISE)
    return posix_madvise(addr, len, advice);
#else
    errno = EINVAL;
    return -1;
#endif
}

> 
> And this is the same case.
> 
> Berto
>