linux-kernel - Re: [RFC PATCH v2 01/16] mm,hwpoison: cleanup unused PageHuge() check

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Date:   Wed, 13 Nov 2019 06:02:03 +0000
From:   Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
CC:     Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 01/16] mm,hwpoison: cleanup unused PageHuge()
 check

On Tue, Nov 12, 2019 at 05:52:58PM +0530, Aneesh Kumar K.V wrote:
> Naoya Horiguchi <n-horiguchi@...jp.nec.com> writes:
> 
> > On Fri, Oct 18, 2019 at 01:48:32PM +0200, Michal Hocko wrote:
> >> On Thu 17-10-19 16:21:08, Oscar Salvador wrote:
> >> > From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> >> > 
> >> > Drop the PageHuge check since memory_failure forks into memory_failure_hugetlb()
> >> > for hugetlb pages.
> >> > 
> >> > Signed-off-by: Oscar Salvador <osalvador@...e.de>
> >> > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> >> 
> >> s-o-b chain is reversed.
> >> 
> >> The code is a bit confusing. Doesn't this check aim for THP?
> >
> > No, PageHuge() is false for thp, so this if branch is just dead code.
> 
> memory_failure()
> {
> 
> 	if (PageTransHuge(hpage)) {
> 		lock_page(p);
> 		if (!PageAnon(p) || unlikely(split_huge_page(p))) {
> 			unlock_page(p);
> 			if (!PageAnon(p))
> 				pr_err("Memory failure: %#lx: non anonymous thp\n",
> 					pfn);
> 			else
> 				pr_err("Memory failure: %#lx: thp split failed\n",
> 					pfn);
> 			if (TestClearPageHWPoison(p))
> 				num_poisoned_pages_dec();
> 			put_hwpoison_page(p);
> 			return -EBUSY;
> 		}
> 		unlock_page(p);
> 		VM_BUG_ON_PAGE(!page_count(p), p);
> 		hpage = compound_head(p);
> 	}
> 
> }
> 
> Do we need that hpage = compund_head(p) conversion there? We should just
> be able to say hpage = p, or even better after this change use p
> directly instead of hpage in the code following?

Thanks for the comment, the target page never be in compound_page
(without races leading to MF_MSG_DIFFERENT_COMPOUND path), so hpage
shouldn't be used afterward.  We also have obsolete comment, so
I feel like the following changes:

  diff --git a/mm/memory-failure.c b/mm/memory-failure.c
  index 392ac277b17d..c9df0f183d6c 100644
  --- a/mm/memory-failure.c
  +++ b/mm/memory-failure.c
  @@ -1319,7 +1319,6 @@ int memory_failure(unsigned long pfn, int flags)
   		}
   		unlock_page(p);
   		VM_BUG_ON_PAGE(!page_count(p), p);
  -		hpage = compound_head(p);
   	}
   
   	/*
  @@ -1391,11 +1390,8 @@ int memory_failure(unsigned long pfn, int flags)
   	/*
   	 * Now take care of user space mappings.
   	 * Abort on fail: __delete_from_page_cache() assumes unmapped page.
  -	 *
  -	 * When the raw error page is thp tail page, hpage points to the raw
  -	 * page after thp split.
   	 */
  -	if (!hwpoison_user_mappings(p, pfn, flags, &hpage)) {
  +	if (!hwpoison_user_mappings(p, pfn, flags, &p)) {
   		action_result(pfn, MF_MSG_UNMAP_FAILED, MF_IGNORED);
   		res = -EBUSY;
   		goto out;

Thanks,
Naoya Horiguchi