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

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Date:   Fri, 5 Feb 2021 11:05:30 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.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 Thu, Feb 04, 2021 at 09:11:24PM -0800, Mike Kravetz wrote:

[...]

> >>> @@ -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?

Right, at least that's my understanding..

> 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().

Yes.  I'll need to take the same lock in v3, so I think I'll just follow.

Thanks,

-- 
Peter Xu