linux-kernel - Re: [PATCH 2/2] mm: Fix a huge pud insertion race during faulting

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Message-Id: <20191118130249.2ec96ffdf9a1aeb4f3e820dd@linux-foundation.org>
Date:   Mon, 18 Nov 2019 13:02:49 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Thomas Hellstrom <thellstrom@...are.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 2/2] mm: Fix a huge pud insertion race during faulting

On Mon, 18 Nov 2019 13:58:04 +0100 Thomas Hellström (VMware) <thomas_os@...pmail.org> wrote:

> >> Is a -stable backport warranted?
> > I believe it is.
> 
> Note that this was caught during a code audit rather than a real 
> experienced problem. It looks to me like the only implementation that 
> currently creates huge pud pagetable entries is dev_dax_huge_fault() 
> which doesn't appear to care much about private (COW) mappings or 
> write-tracking which is, I believe, a prerequisite for create_huge_pud() 
> falling back on thread 1, but not in thread 2.
> 
> This means (assuming that's intentional) that a stable backport 
> shouldn't be needed.
> 
> For the WIP huge page support for graphics memory we'll be allowing both 
> COW mappings and write-tracking, though, but that's still some time away.
> 
> In any case, I think this patch needs -rc testing to catch potential 
> pud_devmap issues before submitted to stable.

OK, thanks, I'll queue it for 5.5-rc1 with a -stable tag.  Hopefully
that way it will get a bit of exposure before the stable trees pick it
up.  Maybe this is optimistic..