Re: [PATCH v3 1/4] osdep: Make qemu_madvise() to set errno in all cases

[Michal Prívozník]

On 6/3/24 10:50, Akihiko Odaki wrote:
> On 2024/06/03 16:56, Michal Prívozník wrote:
>> On 6/2/24 08:26, Akihiko Odaki wrote:
>>> On 2024/06/01 0:46, Philippe Mathieu-Daudé wrote:
>>>> On 31/5/24 17:10, Michal Privoznik wrote:
>>>>> The unspoken premise of qemu_madvise() is that errno is set on
>>>>> error. And it is mostly the case except for posix_madvise() which
>>>>> is documented to return either zero (on success) or a positive
>>>>> error number. This means, we must set errno ourselves. And while
>>>>> at it, make the function return a negative value on error, just
>>>>> like other error paths do.
>>>>>
>>>>> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
>>>>> ---
>>>>>    util/osdep.c | 14 +++++++++++++-
>>>>>    1 file changed, 13 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/util/osdep.c b/util/osdep.c
>>>>> index e996c4744a..1345238a5c 100644
>>>>> --- a/util/osdep.c
>>>>> +++ b/util/osdep.c
>>>>> @@ -57,7 +57,19 @@ int qemu_madvise(void *addr, size_t len, int
>>>>> advice)
>>>>>    #if defined(CONFIG_MADVISE)
>>>>>        return madvise(addr, len, advice);
>>>>>    #elif defined(CONFIG_POSIX_MADVISE)
>>>>> -    return posix_madvise(addr, len, advice);
>>>>> +    /*
>>>>> +     * On Darwin posix_madvise() has the same return semantics as
>>>>> +     * plain madvise, i.e. errno is set and -1 is returned.
>>>>> Otherwise,
>>>>> +     * a positive error number is returned.
>>>>> +     */
>>>>
>>>> Alternative is to guard with #ifdef CONFIG_DARWIN ... #else ... #endif
>>>> which might be clearer.
>>>>
>>>> Although this approach seems reasonable, so:
>>>> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>>
>>> We should use plain madvise() if posix_madvise() is broken. In fact,
>>> QEMU detects the availability of plain madvise() and use it instead of
>>> posix_madvise() on my MacBook.
>>>
>>> Perhaps it may be better to stop defining CONFIG_POSIX_MADVISE on Darwin
>>> to ensure we never use the broken implementation.
>>>
>>
>> Well, doesn't Darwin have madvise() in the first place?
>>
>> https://opensource.apple.com/source/xnu/xnu-7195.81.3/bsd/man/man2/madvise.2.auto.html
>>
>> I thought that's the reason for posix_madvise() to behave the same as
>> madvise() there.
> 
> It does have madvise() and QEMU on my MacBook uses it instead of
> posix_madvise().
> 

I don't have a Mac myself, but I ran some tests on my colleague's Mac
and yes, posix_madvise() is basically just an alias to madvise(). No
dispute there.

> The behavior of posix_madvise() is probably just a bug (and perhaps it
> is too late for them to fix).
> 

So what does this mean for this patch? Should I resend with the change
you're suggesting or this is good as is? I mean, posix_madvise() is not
going to be used on Mac anyways.

Michal