Re: [mm-unstable PATCH v6 3/8] mm, hwpoison, hugetlb: support saving mechanism of raw error pages

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Wed, Jul 13, 2022 at 12:51:30PM +0800, Miaohe Lin wrote:
> On 2022/7/12 11:28, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > 
> > When handling memory error on a hugetlb page, the error handler tries to
> > dissolve and turn it into 4kB pages.  If it's successfully dissolved,
> > PageHWPoison flag is moved to the raw error page, so that's all right.
> > However, dissolve sometimes fails, then the error page is left as
> > hwpoisoned hugepage. It's useful if we can retry to dissolve it to save
> > healthy pages, but that's not possible now because the information about
> > where the raw error pages is lost.
> > 
> > Use the private field of a few tail pages to keep that information.  The
> > code path of shrinking hugepage pool uses this info to try delayed dissolve.
> > In order to remember multiple errors in a hugepage, a singly-linked list
> > originated from SUBPAGE_INDEX_HWPOISON-th tail page is constructed.  Only
> > simple operations (adding an entry or clearing all) are required and the
> > list is assumed not to be very long, so this simple data structure should
> > be enough.
> > 
> > If we failed to save raw error info, the hwpoison hugepage has errors on
> > unknown subpage, then this new saving mechanism does not work any more,
> > so disable saving new raw error info and freeing hwpoison hugepages.
> > 
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > Reported-by: kernel test robot <lkp@intel.com>
> 
> This patch looks good to me with some nits below.
> 
> > ---
> ...
> > +static int hugetlb_set_page_hwpoison(struct page *hpage, struct page *page)
> > +{
> > +	struct llist_head *head;
> > +	struct raw_hwp_page *raw_hwp;
> > +	struct llist_node *t, *tnode;
> > +	int ret = TestSetPageHWPoison(hpage) ? -EHWPOISON : 0;
> > +
> > +	/*
> > +	 * Once the hwpoison hugepage has lost reliable raw error info,
> > +	 * there is little meaning to keep additional error info precisely,
> > +	 * so skip to add additional raw error info.
> > +	 */
> > +	if (HPageRawHwpUnreliable(hpage))
> > +		return -EHWPOISON;
> > +	head = raw_hwp_list_head(hpage);
> > +	llist_for_each_safe(tnode, t, head->first) {
> > +		struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
> > +
> > +		if (p->page == page)
> > +			return -EHWPOISON;
> > +	}
> > +
> > +	raw_hwp = kmalloc(sizeof(struct raw_hwp_page), GFP_ATOMIC);
> > +	if (raw_hwp) {
> > +		raw_hwp->page = page;
> > +		llist_add(&raw_hwp->node, head);
> > +		/* the first error event will be counted in action_result(). */
> > +		if (ret)
> > +			num_poisoned_pages_inc();
> > +	} else {
> > +		/*
> > +		 * Failed to save raw error info.  We no longer trace all
> > +		 * hwpoisoned subpages, and we need refuse to free/dissolve
> > +		 * this hwpoisoned hugepage.
> > +		 */
> > +		SetHPageRawHwpUnreliable(hpage);
> 
> IMHO, when HPageRawHwpUnreliable is set, we can simply free the raw_hwp_list here
> to save some memory as they're not used anymore.

Yes, that's possible. Let's do this.

> 
> > +	}
> > +	return ret;
> > +}
> > +
> > +void hugetlb_clear_page_hwpoison(struct page *hpage)
> > +{
> > +	struct llist_head *head;
> > +	struct llist_node *t, *tnode;
> > +
> > +	if (!HPageRawHwpUnreliable(hpage))
> > +		ClearPageHWPoison(hpage);
> > +	head = raw_hwp_list_head(hpage);
> > +	llist_for_each_safe(tnode, t, head->first) {
> > +		struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
> > +
> > +		SetPageHWPoison(p->page);
> > +		kfree(p);
> > +	}
> > +	llist_del_all(head);
> > +}
> > +
> >  /*
> >   * Called from hugetlb code with hugetlb_lock held.
> >   *
> > @@ -1698,7 +1771,7 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
> >  		goto out;
> >  	}
> >  
> > -	if (TestSetPageHWPoison(head)) {
> > +	if (hugetlb_set_page_hwpoison(head, page)) {
> >  		ret = -EHWPOISON;
> >  		goto out;
> >  	}
> > @@ -1710,7 +1783,6 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
> >  	return ret;
> >  }
> >  
> > -#ifdef CONFIG_HUGETLB_PAGE
> >  /*
> >   * Taking refcount of hugetlb pages needs extra care about race conditions
> >   * with basic operations like hugepage allocation/free/demotion.
> > @@ -1751,7 +1823,7 @@ static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb
> >  	lock_page(head);
> >  
> >  	if (hwpoison_filter(p)) {
> > -		ClearPageHWPoison(head);
> > +		hugetlb_clear_page_hwpoison(head);
> 
> If the code reach here, the hugetlb page should be poisoned first time or it will return
> in above "res == -EHWPOISON" case. So ClearPageHWPoison should be fine here. But this change
> will do the same thing while making the code more consistent. So it should be fine. :)

Right, this change is for consistency.

> 
> >  		res = -EOPNOTSUPP;
> >  		goto out;
> >  	}
> > 
> 
> Anyway, for what it worth,
> 
> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
> 
> Thanks!

Thank you.
- Naoya Horiguchi