linux-kernel - Re: [PATCH v3 2/2] mm,hwpoison: make get_hwpoison_page call get_any

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Date:   Thu, 13 May 2021 00:03:04 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Oscar Salvador <osalvador@...e.de>
CC:     Naoya Horiguchi <nao.horiguchi@...il.com>,
        Muchun Song <songmuchun@...edance.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Tony Luck <tony.luck@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] mm,hwpoison: make get_hwpoison_page call
 get_any_page()

On Wed, May 12, 2021 at 10:55:22AM +0200, Oscar Salvador wrote:
> On Wed, May 12, 2021 at 12:10:16AM +0900, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@....com>
> > 
> > Now __get_hwpoison_page() could return -EBUSY in a race condition,
> > so it's helpful to handle it by retrying.  We already have retry
> > logic, so make get_hwpoison_page() call get_any_page() when called
> > from memory_failure().
> 
> As I stated in your previous patch, I think you should start returning -EBUSY
> from this patch onwards.
> 
> >  static int get_any_page(struct page *p, unsigned long flags)
> >  {
> >  	int ret = 0, pass = 0;
> > @@ -1152,50 +1136,76 @@ static int get_any_page(struct page *p, unsigned long flags)
> >  		count_increased = true;
> >  
> >  try_again:
> > -	if (!count_increased && !__get_hwpoison_page(p)) {
> > -		if (page_count(p)) {
> > -			/* We raced with an allocation, retry. */
> > -			if (pass++ < 3)
> > -				goto try_again;
> > -			ret = -EBUSY;
> > -		} else if (!PageHuge(p) && !is_free_buddy_page(p)) {
> > -			/* We raced with put_page, retry. */
> > -			if (pass++ < 3)
> > -				goto try_again;
> > -			ret = -EIO;
> > +	if (!count_increased) {
> > +		ret = __get_hwpoison_page(p);
> > +		if (!ret) {
> > +			if (page_count(p)) {
> > +				/* We raced with an allocation, retry. */
> > +				if (pass++ < 3)
> > +					goto try_again;
> > +				ret = -EBUSY;
> > +			} else if (!PageHuge(p) && !is_free_buddy_page(p)) {
> > +				/* We raced with put_page, retry. */
> > +				if (pass++ < 3)
> > +					goto try_again;
> > +				ret = -EIO;
> > +			}
> > +			goto out;
> >  		}
> > +	}
> 
> I think this can be improved.
> 
> We cannot have -EBUSY unless we come from __get_hwpoison_page() (!count_increased),
> so I think a much more natural approach would be to stuff the hunk below in there,
> and then place the other stuff in an else, so something like:
> 
>         if (!count_increased) {
>                 ret = __get_hwpoison_page(p);
>                 if (!ret) {
>                         if (page_count(p)) {
>                                 /* We raced with an allocation, retry. */
>                                 if (pass++ < 3)
>                                         goto try_again;
>                                 ret = -EBUSY;
>                         } else if (!PageHuge(p) && !is_free_buddy_page(p)) {
>                                 /* We raced with put_page, retry. */
>                                 if (pass++ < 3)
>                                         goto try_again;
>                                 ret = -EIO;
>                         }
>                         goto out;
>                 } else if (ret == -EBUSY) {
> 			/* We raced with freeing huge page to buddy, retry. */
> 			if (pass++ < 3)
> 				goto try_again;
> 		}

Moving "if (ret == -EBUSY)" block to here makes sense to me. Thank you.

>         } else {

In the original logic, if __get_hwpoison_page() returns 1, we fall into the
"if (PageHuge(p) || PageLRU(p) || __PageMovable(p)" check.  I guess that this
"else" seems not necessary?

> 		/* We do already have a refcount for this page, see if we can
> 		 * handle it.
> 		 */
> 		if (PageHuge(p) || PageLRU(p) || __PageMovable(p)) {
> 			ret = 1;
> 		} else {
> 			/* A page we cannot handle. Check...
> 		}
> 	}

Thanks,
Naoya Horiguchi