Re: [PATCH 1/4] spapr: qemu_memalign() doesn't return NULL

[Greg Kurz <groug@kaod.org>]

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On Tue, 27 Oct 2020 12:56:40 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Oct 26, 2020 at 03:46:47PM +0100, Greg Kurz wrote:
> > On Mon, 26 Oct 2020 14:43:08 +0100
> > Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
> > 
> > > On 10/26/20 1:40 PM, Greg Kurz wrote:
> > > > qemu_memalign() aborts if OOM. Drop some dead code.
> > > > 
> > > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > > ---
> > > >   hw/ppc/spapr.c       |    6 ------
> > > >   hw/ppc/spapr_hcall.c |    8 ++------
> > > >   2 files changed, 2 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > > index 0cc19b5863a4..f098d0ee6d98 100644
> > > > --- a/hw/ppc/spapr.c
> > > > +++ b/hw/ppc/spapr.c
> > > > @@ -1521,12 +1521,6 @@ void spapr_reallocate_hpt(SpaprMachineState *spapr, int shift,
> > > >           int i;
> > > >   
> > > >           spapr->htab = qemu_memalign(size, size);
> > > > -        if (!spapr->htab) {
> > > > -            error_setg_errno(errp, errno,
> > > > -                             "Could not allocate HPT of order %d", shift);
> > > > -            return;
> > > 
> > > Wasn't the idea to use qemu_try_memalign() here?
> > > 
> > 
> > Well... I have mixed feeling around this. The HTAB was first
> > introduced by commit:
> > 
> > commit f43e35255cffb6ac6230dd09d308f7909f823f96
> > Author: David Gibson <david@gibson.dropbear.id.au>
> > Date:   Fri Apr 1 15:15:22 2011 +1100
> > 
> >     Virtual hash page table handling on pSeries machine
> > 
> > using qemu_mallocz(), which was aborting on OOM. It then got
> > replaced by g_malloc0() when qemu_mallocz() got deprecated
> > and eventually by qemu_memalign() when KVM support was added.
> > 
> > Surviving OOM when allocating the HTAB never seemed to be an
> > option until this commit that introduced the check:
> > 
> > commit c5f54f3e31bf693f70a98d4d73ea5dbe05689857
> > Author: David Gibson <david@gibson.dropbear.id.au>
> > Date:   Tue Feb 9 10:21:56 2016 +1000
> > 
> >     pseries: Move hash page table allocation to reset time
> > 
> > I don't really see in the patch and in the changelog an obvious
> > desire to try to handle OOM.
> 
> 
> This one is probably ok.  AFAICT all failures returned here would be
> more or less fatal in the caller, one way or another (&error_fatal in
> two cases, and failure to load an incoming migration stream in the
> other).
> 
> > > > -        }
> > > > -
> > > >           memset(spapr->htab, 0, size);
> > > >           spapr->htab_shift = shift;
> > > >   
> > > > diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> > > > index 607740150fa2..34e146f628fb 100644
> > > > --- a/hw/ppc/spapr_hcall.c
> > > > +++ b/hw/ppc/spapr_hcall.c
> > > > @@ -361,12 +361,8 @@ static void *hpt_prepare_thread(void *opaque)
> > > >       size_t size = 1ULL << pending->shift;
> > > >   
> > > >       pending->hpt = qemu_memalign(size, size);
> > > > -    if (pending->hpt) {
> > > > -        memset(pending->hpt, 0, size);
> > > > -        pending->ret = H_SUCCESS;
> > > > -    } else {
> > > > -        pending->ret = H_NO_MEM;
> > > 
> > > Ditto.
> > > 
> > 
> > This one was introduced by commit:
> > 
> > commit 0b0b831016ae93bc14698a5d7202eb77feafea75
> > Author: David Gibson <david@gibson.dropbear.id.au>
> > Date:   Fri May 12 15:46:49 2017 +1000
> > 
> >     pseries: Implement HPT resizing
> > 
> > I agree that maybe the intent here could have been to use qemu_try_memalign(),
> > but again I don't quite see any strong justification to handle OOM in the
> > changelog.
> > 
> > David,
> > 
> > Any insight to share ?
> 
> Aborting on an HPT resize failure is definitely not ok, though.  This
> one needs to be a qemu_try_memalign().
> 

Ok, I'll fix that.

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