linux-kernel - Re: [PATCH 0/4] mm/hugetlb: Early cow on fork, and a few cleanups

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Date:   Thu, 4 Feb 2021 16:32:45 +0200
From:   Gal Pressman <galpress@...zon.com>
To:     Peter Xu <peterx@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>
CC:     Wei Zhang <wzam@...zon.com>, Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Kirill Shutemov <kirill@...temov.name>,
        David Gibson <david@...son.dropbear.id.au>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] mm/hugetlb: Early cow on fork, and a few cleanups

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.