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

[Greg Kurz <groug@kaod.org>]

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.

> > -        }
> > -
> >           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 ?

> > -    }
> > +    memset(pending->hpt, 0, size);
> > +    pending->ret = H_SUCCESS;
> >   
> >       qemu_mutex_lock_iothread();
> >   
>