Re: [PATCH] softmmu/physmem: Use qemu_madvise

[David Hildenbrand]

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.

-- 
Thanks,

David / dhildenb