linux-kernel - Re: [PATCH 14/31] fs/userfaultfd: retry if pte_offset

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Message-ID: <ZG6QNhFhyNxyn5PK@x1n>
Date:   Wed, 24 May 2023 18:31:18 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 14/31] fs/userfaultfd: retry if pte_offset_map() fails

On Sun, May 21, 2023 at 10:06:32PM -0700, Hugh Dickins wrote:
> Instead of worrying whether the pmd is stable, userfaultfd_must_wait()
> call pte_offset_map() as before, but go back to try again if that fails.
> 
> Risk of endless loop?  It already broke out if pmd_none(), !pmd_present()
> or pmd_trans_huge(), and pte_offset_map() would have cleared pmd_bad():
> which leaves pmd_devmap().  Presumably pmd_devmap() is inappropriate in
> a vma subject to userfaultfd (it would have been mistreated before),
> but add a check just to avoid all possibility of endless loop there.

Agreed, afaiu that's for either PFNMAP or MIXEDMAP vmas only.  Maybe we can
use a WARN_ON_ONCE() for that to be clear, but no strong opinions.

> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Acked-by: Peter Xu <peterx@...hat.com>

-- 
Peter Xu