linux-kernel - Re: [PATCH v2 4/4] hugetlb: Do early cow when page pinned on src mm

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Date:   Thu, 4 Feb 2021 21:11:24 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Kirill Shutemov <kirill@...temov.name>,
        Wei Zhang <wzam@...zon.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Gal Pressman <galpress@...zon.com>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        David Gibson <david@...son.dropbear.id.au>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 4/4] hugetlb: Do early cow when page pinned on src mm

On 2/4/21 5:43 PM, Peter Xu wrote:
> On Thu, Feb 04, 2021 at 03:25:37PM -0800, Mike Kravetz wrote:
>> On 2/4/21 6:50 AM, Peter Xu wrote:
>>> This is the last missing piece of the COW-during-fork effort when there're
>>> pinned pages found.  One can reference 70e806e4e645 ("mm: Do early cow for
>>> pinned pages during fork() for ptes", 2020-09-27) for more information, since
>>> we do similar things here rather than pte this time, but just for hugetlb.
>>>
>>> Signed-off-by: Peter Xu <peterx@...hat.com>
>>> ---
>>>  mm/hugetlb.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++-----
>>>  1 file changed, 56 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 9e6ea96bf33b..5793936e00ef 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> +	__SetPageUptodate(new_page);
>>> +	ClearPagePrivate(new_page);
>>> +	set_page_huge_active(new_page);
>>
>> Code to replace the above ClearPagePrivate and set_page_huge_active is
>> in Andrew's tree.  With changes in Andrew's tree, this would be:
>>
>> 	ClearHPageRestoreReserve(new_page);
>> 	SetHPageMigratable(new_page);
> 
> Indeed these names are much better than using the default ones.  At the
> meantime I'll rebase to linux-next/akpm.  Sorry it's always not easy for me to
> find the right branch...

No worries.  I only know because I recently changed these.

...
>>> @@ -3787,7 +3803,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>>>  		dst_entry = huge_ptep_get(dst_pte);
>>>  		if ((dst_pte == src_pte) || !huge_pte_none(dst_entry))
>>>  			continue;
>>> -
>>> +again:
>>>  		dst_ptl = huge_pte_lock(h, dst, dst_pte);
>>>  		src_ptl = huge_pte_lockptr(h, src, src_pte);
>>>  		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> 
> Side question: Mike, do you know why we need this lock_nested()?  Could the src
> lock be taken due to any reason already?  It confused me when I read the chunk.

I see that it was added with commit 4647875819aa.  That was when huge pages
used the single per-mm ptl.  Lockdep seemed to complain about taking
&mm->page_table_lock twice.   Certainly, source and destination mm can not
be the same.  Right?  I do not have the full history, but it 'looks' like
lockdep might have been confused and this was added to keep it quiet.

BTW - Copy page range for 'normal' pages has the same spin_lock_nested().
-- 
Mike Kravetz