linux-kernel - Re: [PATCH -next] mm: hwpoison: support recovery from HugePage copy-on-write faults

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Date:   Wed, 12 Apr 2023 14:57:18 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Liu Shixin <liushixin2@...wei.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Tony Luck <tony.luck@...el.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm: hwpoison: support recovery from HugePage
 copy-on-write faults

On Wed, 12 Apr 2023 11:13:50 -0700 Mike Kravetz <mike.kravetz@...cle.com> wrote:

> On 04/11/23 17:27, Liu Shixin wrote:
> > Patch a873dfe1032a ("mm, hwpoison: try to recover from copy-on write faults")
> > introduced a new copy_user_highpage_mc() function, and fix the kernel crash
> > when the kernel is copying a normal page as the result of a copy-on-write
> > fault and runs into an uncorrectable error. But it doesn't work for HugeTLB.
> 
> Andrew asked about user-visible effects.  Perhaps, a better way of
> stating this in the commit message might be:
> 
> Commit a873dfe1032a ("mm, hwpoison: try to recover from copy-on write
> faults") introduced the routine copy_user_highpage_mc() to gracefully
> handle copying of user pages with uncorrectable errors.  Previously,
> such copies would result in a kernel crash.  hugetlb has separate code
> paths for copy-on-write and does not benefit from the changes made in
> commit a873dfe1032a.
> 
> Modify hugetlb copy-on-write code paths to use copy_mc_user_highpage()
> so that they can also gracefully handle uncorrectable errors in user
> pages.  This involves changing the hugetlb specific routine
> ?copy_user_folio()? from type void to int so that it can return an error.
> Modify the hugetlb userfaultfd code in the same way so that it can return
> -EHWPOISON if it encounters an uncorrectable error.

Thanks, but...  what are the runtime effects?  What does hugetlb
presently do when encountering these uncorrectable error?