[PATCH v3 1/2] mm: cma: fix allocation may fail sometimes

[Dong Aisheng]

On Wed, Mar 16, 2022 at 6:58 AM Andrew Morton <akpm at linux-foundation.org> wrote:
>
> On Tue, 15 Mar 2022 22:45:20 +0800 Dong Aisheng <aisheng.dong at nxp.com> wrote:
>
> > --- a/mm/cma.c
> > +++ b/mm/cma.c
> >
> > ...
> >
> > @@ -457,6 +458,16 @@ struct page *cma_alloc(struct cma *cma, unsigned long count,
> >                               offset);
> >               if (bitmap_no >= bitmap_maxno) {
> >                       spin_unlock_irq(&cma->lock);
> > +                     pr_debug("%s(): alloc fail, retry loop %d\n", __func__, loop++);
> > +                     /*
> > +                      * rescan as others may finish the memory migration
> > +                      * and quit if no available CMA memory found finally
> > +                      */
> > +                     if (start) {
> > +                             schedule();
> > +                             start = 0;
> > +                             continue;
> > +                     }
> >                       break;
>
> The schedule() is problematic. For a start, we'd normally use
> cond_resched() here, so we avoid calling the more expensive schedule()
> if we know it won't perform any action.
>
> But cond_resched() is problematic if this thread has realtime
> scheduling policy and the process we're waiting on does not.  One way
> to address that is to use an unconditional msleep(1), but that's still
> just a hack.
>

I think we can simply drop schedule() here during the second round of retry
as the estimated delay may not be really needed.

Do you think that's ok?

> A much cleaner solution is to use appropriate locking so that various
> threads run this code in order, without messing each other up.
>
> And it looks like the way to do that is to simply revert the commit
> which caused this regression, a4efc174b382 ("mm/cma.c: remove redundant
> cma_mutex lock")?

Yes, agree it could be a backup solution if not better ideas.

Regards
Aisheng