linux-kernel - Re: [RFC PATCH v2 34/47] hugetlb: userfaultfd: add support for high-granularity UFFDIO

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Date:   Fri, 23 Dec 2022 13:38:02 -0500
From:   Peter Xu <peterx@...hat.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 34/47] hugetlb: userfaultfd: add support for
 high-granularity UFFDIO_CONTINUE

James,

On Fri, Oct 21, 2022 at 04:36:50PM +0000, James Houghton wrote:
> +	bool use_hgm = uffd_ctx_has_hgm(&dst_vma->vm_userfaultfd_ctx) &&
> +		mode == MCOPY_ATOMIC_CONTINUE;

Do you think in your new version use_hgm can work even for MISSING by
default?

I had a feeling that the major components are ready for that anyway.  Then
no matter how HGM is enabled (assuming it'll switch to MADV, or even one
can just register with MISSING+MINOR and enable the uffd HGM feature), an
existing MISSING only app can easily switch to HGM support if it's on huge
pages.

Thanks,

-- 
Peter Xu