Re: [PATCH] softmmu/physmem: Use qemu_madvise

[Dr. David Alan Gilbert]

* Andrew Deason (adeason@sinenomine.net) 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?

For ballooning we don't really need Linux semantics; we just need it to
use less host memory.  Postcopy needs the more careful semantics though.

Dave
> -- 
> Andrew Deason
> adeason@sinenomine.net
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK