Re: [PATCH v2 4/9] mm, hwpoison, hugetlb: support saving mechanism of raw error pages

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

On Mon, Jun 27, 2022 at 11:16:06AM +0800, Miaohe Lin wrote:
> On 2022/6/24 7:51, 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>
> 
> Many thanks for your patch. This patch looks good to me. Some nits below.
> 
> >
> <snip>
> >
> > +static inline 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;
> > +
> > +	/*
> > +	 * Once the hwpoison hugepage has lost reliable raw error info,
> > +	 * there is little mean to keep additional error info precisely,
> 
> It should be s/mean/meaning/ ?

Right, fixed.

> 
> > +	 * so skip to add additional raw error info.
> > +	 */
> > +	if (raw_hwp_unreliable(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;
> > +	}
> > +
> > +	ret = TestSetPageHWPoison(hpage) ? -EHWPOISON : 0;
> > +	/* the first error event will be counted in action_result(). */
> > +	if (ret)
> > +		num_poisoned_pages_inc();
> > +
> > +	raw_hwp = kmalloc(sizeof(struct raw_hwp_page), GFP_KERNEL);
> > +	if (raw_hwp) {
> > +		raw_hwp->page = page;
> > +		llist_add(&raw_hwp->node, head);
> > +	} else {
> > +		/*
> > +		 * Failed to save raw error info.  We no longer trace all
> > +		 * hwpoisoned subpages, and we need refuse to free/dissolve
> > +		 * this hwpoisoned hugepage.
> > +		 */
> > +		set_raw_hwp_unreliable(hpage);
> > +		return ret;
> 
> This "return ret" can be combined into the below one?
> 

Right, fixed.

> > +	}
> > +	return ret;
> > +}
> > +
> > +inline int hugetlb_clear_page_hwpoison(struct page *hpage)
> > +{
> > +	struct llist_head *head;
> > +	struct llist_node *t, *tnode;
> > +
> > +	if (raw_hwp_unreliable(hpage))
> > +		return -EBUSY;
> 
> Can we try freeing the memory of raw_hwp_list to save possible memory? It seems
> raw_hwp_list becomes unneeded when raw_hwp_unreliable.

Yes, we can. I try to change like that.
Thanks for the suggestion.

- Naoya Horiguchi