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Subject: Re: [PATCH 0/4] mm/hugetlb: Early cow on fork, and a few cleanups
To:     Peter Xu <peterx@redhat.com>, <linux-kernel@vger.kernel.org>,
        <linux-mm@kvack.org>
CC:     Wei Zhang <wzam@amazon.com>, Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, Christoph Hellwig <hch@lst.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Jan Kara <jack@suse.cz>,
        Kirill Shutemov <kirill@shutemov.name>,
        David Gibson <david@gibson.dropbear.id.au>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20210203210832.113685-1-peterx@redhat.com>
From:   Gal Pressman <galpress@amazon.com>
Message-ID: <de213e3e-01c7-0fef-d5cf-6a69ec670c70@amazon.com>
Date:   Thu, 4 Feb 2021 16:32:45 +0200
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On 03/02/2021 23:08, Peter Xu wrote:
> As reported by Gal [1], we still miss the code clip to handle early cow for
> 
> hugetlb case, which is true.  Again, it still feels odd to fork() after using a
> 
> few huge pages, especially if they're privately mapped to me..  However I do
> 
> agree with Gal and Jason in that we should still have that since that'll
> 
> complete the early cow on fork effort at least, and it'll still fix issues
> 
> where buffers are not well under control and not easy to apply MADV_DONTFORK.
> 
> 
> 
> The first two patches (1-2) are some cleanups I noticed when reading into the
> 
> hugetlb reserve map code.  I think it's good to have but they're not necessary
> 
> for fixing the fork issue.
> 
> 
> 
> The last two patches (3-4) is the real fix.
> 
> 
> 
> I tested this with a fork() after some vfio-pci assignment, so I'm pretty sure
> 
> the page copy path could trigger well (page will be accounted right after the
> 
> fork()), but I didn't do data check since the card I assigned is some random
> 
> nic.  Gal, please feel free to try this if you have better way to verify the
> 
> series.

Thanks Peter, once v2 is submitted I'll pull the patches and we'll run the tests
that discovered the issue to verify it works.