Re: [PATCH v1 2/2] mm: soft-offline: close the race against page allocation

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Fri, Jul 13, 2018 at 01:40:02PM -0700, Andrew Morton wrote:
> On Fri, 13 Jul 2018 12:26:06 +0900 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:
> 
> > A process can be killed with SIGBUS(BUS_MCEERR_AR) when it tries to
> > allocate a page that was just freed on the way of soft-offline.
> > This is undesirable because soft-offline (which is about corrected error)
> > is less aggressive than hard-offline (which is about uncorrected error),
> > and we can make soft-offline fail and keep using the page for good reason
> > like "system is busy."
> > 
> > Two main changes of this patch are:
> > 
> > - setting migrate type of the target page to MIGRATE_ISOLATE. As done
> >   in free_unref_page_commit(), this makes kernel bypass pcplist when
> >   freeing the page. So we can assume that the page is in freelist just
> >   after put_page() returns,
> > 
> > - setting PG_hwpoison on free page under zone->lock which protects
> >   freelists, so this allows us to avoid setting PG_hwpoison on a page
> >   that is decided to be allocated soon.
> > 
> >
> > ...
> >
> > +
> > +#ifdef CONFIG_MEMORY_FAILURE
> > +/*
> > + * Set PG_hwpoison flag if a given page is confirmed to be a free page
> > + * within zone lock, which prevents the race against page allocation.
> > + */
> 
> I think this is clearer?
> 
> --- a/mm/page_alloc.c~mm-soft-offline-close-the-race-against-page-allocation-fix
> +++ a/mm/page_alloc.c
> @@ -8039,8 +8039,9 @@ bool is_free_buddy_page(struct page *pag
>  
>  #ifdef CONFIG_MEMORY_FAILURE
>  /*
> - * Set PG_hwpoison flag if a given page is confirmed to be a free page
> - * within zone lock, which prevents the race against page allocation.
> + * Set PG_hwpoison flag if a given page is confirmed to be a free page.  This
> + * test is performed under the zone lock to prevent a race against page
> + * allocation.

Yes, I like it.

Thanks,
Naoya Horiguchi