Re: [PATCH] softmmu/physmem: Use qemu_madvise

[Peter Maydell]

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.

-- PMM