Re: [PATCH] softmmu/physmem: Use qemu_madvise

[David Hildenbrand <david@redhat.com>]

On 22.03.22 17:39, Andrew Deason wrote:
> On Wed, 16 Mar 2022 10:41:41 +0100
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 16.03.22 10:37, Dr. David Alan Gilbert wrote:
>>> * Peter Maydell (peter.maydell@linaro.org) wrote:
>>>> On Wed, 16 Mar 2022 at 07:53, David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>> On 16.03.22 05:04, Andrew Deason wrote:
>>>>>> We have a thin wrapper around madvise, called qemu_madvise, which
>>>>>> provides consistent behavior for the !CONFIG_MADVISE case, and works
>>>>>> around some platform-specific quirks (some platforms only provide
>>>>>> posix_madvise, and some don't offer all 'advise' types). This specific
>>>>>> caller of madvise has never used it, tracing back to its original
>>>>>> introduction in commit e0b266f01dd2 ("migration_completion: Take
>>>>>> current state").
>>>>>>
>>>>>> Call qemu_madvise here, to follow the same logic as all of our other
>>>>>> madvise callers. This slightly changes the behavior for
>>>>>> !CONFIG_MADVISE (EINVAL instead of ENOSYS, and a slightly different
>>>>>> error message), but this is now more consistent with other callers
>>>>>> that use qemu_madvise.
>>>>>>
>>>>>> Signed-off-by: Andrew Deason <adeason@sinenomine.net>
>>>>>> ---
>>>>>> Looking at the history of commits that touch this madvise() call, it
>>>>>> doesn't _look_ like there's any reason to be directly calling madvise vs
>>>>>> qemu_advise (I don't see anything mentioned), but I'm not sure.
>>>>>>
>>>>>>  softmmu/physmem.c | 12 ++----------
>>>>>>  1 file changed, 2 insertions(+), 10 deletions(-)
>>>>>>
>>>>>> diff --git a/softmmu/physmem.c b/softmmu/physmem.c
>>>>>> index 43ae70fbe2..900c692b5e 100644
>>>>>> --- a/softmmu/physmem.c
>>>>>> +++ b/softmmu/physmem.c
>>>>>> @@ -3584,40 +3584,32 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
>>>>>>                           rb->idstr, start, length, ret);
>>>>>>              goto err;
>>>>>>  #endif
>>>>>>          }
>>>>>>          if (need_madvise) {
>>>>>>              /* For normal RAM this causes it to be unmapped,
>>>>>>               * for shared memory it causes the local mapping to disappear
>>>>>>               * and to fall back on the file contents (which we just
>>>>>>               * fallocate'd away).
>>>>>>               */
>>>>>> -#if defined(CONFIG_MADVISE)
>>>>>>              if (qemu_ram_is_shared(rb) && rb->fd < 0) {
>>>>>> -                ret = madvise(host_startaddr, length, QEMU_MADV_REMOVE);
>>>>>> +                ret = qemu_madvise(host_startaddr, length, QEMU_MADV_REMOVE);
>>>>>>              } else {
>>>>>> -                ret = madvise(host_startaddr, length, QEMU_MADV_DONTNEED);
>>>>>> +                ret = qemu_madvise(host_startaddr, length, QEMU_MADV_DONTNEED);
>>>>>
>>>>> posix_madvise(QEMU_MADV_DONTNEED) has completely different semantics
>>>>> then madvise() -- it's not a discard that we need here.
>>>>>
>>>>> So ram_block_discard_range() would now succeed in environments (BSD?)
>>>>> where it's supposed to fail.
>>>>>
>>>>> So AFAIKs this isn't sane.
>>>>
>>>> But CONFIG_MADVISE just means "host has madvise()"; it doesn't imply
>>>> "this is a Linux madvise() with MADV_DONTNEED". Solaris madvise()
>>>> doesn't seem to have  MADV_DONTNEED at all; a quick look at the
>>>> FreeBSD manpage suggests its madvise MADV_DONTNEED is identical
>>>> to its posix_madvise MADV_DONTNEED.
>>>>
>>>> If we need "specifically Linux MADV_DONTNEED semantics" maybe we
>>>> should define a QEMU_MADV_LINUX_DONTNEED which either (a) does the
>>>> right thing or (b) fails, and use qemu_madvise() regardless.
>>>>
>>>> Certainly the current code is pretty fragile to being changed by
>>>> people who don't understand the undocumented subtlety behind
>>>> the use of a direct madvise() call here.
>>>
>>> Yeh and I'm not sure I can remembe rall the subtleties; there's a big
>>> hairy set of ifdef's in include/qemu/madvise.h that makes
>>> sure we always have the definition of QEMU_MADV_REMOVE/DONTNEED
>>> even on platforms that might not define it themselves.
>>>
>>> But I think this code is used for things with different degrees
>>> of care about the semantics; e.g. 'balloon' just cares that
>>> it frees memory up and doesn't care about the detailed semantics
>>> that much; so it's probably fine with that.
>>> Postcopy is much more touchy, but then it's only going to be
>>> calling this on Linux anyway (because of the userfault dependency).
>>
>> MADV_DONTNEED/MADV_REMOVE only provides discard semantics on Linux IIRC
>> -- and that's what we want to achieve: ram_block_discard_range()
>>
>> So I agree with Peter that we might want to make this more explicit.
> 
> I was looking at the comments/history around this code to try to make
> this more explicit/clear, and it seems like the whole function is very
> Linux-specific. All we ever do is:
> 
> - fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE)
> - madvise(MADV_REMOVE)
> - madvise(MADV_DONTNEED) with Linux semantics
> 
> All of those operations are Linux-only, so trying to figure out the
> cross-platform way to model this seems kind of pointless. Is it fine to
> just #ifdef the whole thing to be just for Linux?
> 

Fine with me, as long as it compiles on other OSs :)

-- 
Thanks,

David / dhildenb