linux-kernel - Re: [PATCH] mm: hugetlbfs: add hwcrp_hugepages to record memory failure on hugetlbfs

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Message-ID: <20210608080112.GA942548@hori.linux.bs1.fc.nec.co.jp>
Date:   Tue, 8 Jun 2021 08:01:12 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     wangbin <wangbin224@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Naoya Horiguchi <nao.horiguchi@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "wuxu.wu@...wei.com" <wuxu.wu@...wei.com>
Subject: Re: [PATCH] mm: hugetlbfs: add hwcrp_hugepages to record memory
 failure on hugetlbfs

Thanks for forwarding the message, Mike.

On Mon, Jun 07, 2021 at 12:13:03PM -0700, Mike Kravetz wrote:
> Resend with new e-mail for Naoya
> 
> On 6/7/21 7:16 AM, wangbin wrote:
> > From: Bin Wang <wangbin224@...wei.com>
> > 
> > In the current hugetlbfs memory failure handler, reserved huge page
> > counts are used to record the number of huge pages with hwposion.
> 
> I do not believe this is an accurate statement.  Naoya is the memory
> error expert and may disagree, but I do not see anywhere where reserve
> counts are being used to track huge pages with memory errors.

And Mike is right, hugetlb's reservation count is not linked
to accounting of hwpoisoned pages.

> 
> IIUC, the routine hugetlbfs_error_remove_page is called after
> unmapping the page from all user mappings.  The routine will simply,
> remove the page from the cache.  This effectively removes the page
> from the file as hugetlbfs is a memory only filesystem.  The subsequent
> call to hugetlb_unreserve_pages cleans up any reserve map entries
> associated with the page and adjusts the reserve count if necessary.
> The reserve count adjustment is based on removing the page from the
> file, rather than the memory error.  The same adjustment would be made
> if the page was hole punched from the file.

This logic totally makes sense to me.

Unmapping done in memory_failure() might increment the reserve count,
but that's the cancel of the consumed reservation by unmapping.

Thanks,
Naoya Horigcuhi