linux-kernel - Re: [RFC PATCH 00/16] 1GB THP support on x86

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Date:   Thu, 3 Sep 2020 17:55:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Nellans <dnellans@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/16] 1GB THP support on x86_64

On Thu, Sep 03, 2020 at 01:40:32PM -0300, Jason Gunthorpe wrote:
> However if the sizeof(*pXX) is 8 on a 32 bit platform then load
> tearing is a problem. At lest the various pXX_*() test functions
> operate on a single 32 bit word so don't tear, but to to convert the
> *pXX to a lower level page table pointer a coherent, untorn, read is
> required.
> 
> So, looking again, I remember now, I could never quite figure out why
> gup_pmd_range() was safe to do:
> 
>                 pmd_t pmd = READ_ONCE(*pmdp);
> [..]
>                 } else if (!gup_pte_range(pmd, addr, next, flags, pages, nr))
> [..]
>         ptem = ptep = pte_offset_map(&pmd, addr);
> 
> As I don't see what prevents load tearing a 64 bit pmd.. Eg no
> pmd_trans_unstable() or equivalent here.

I don't think there are any 32-bit page tables which support a PUD-sized
page.  Pretty sure x86 doesn't until you get to 4- or 5- level page tables
(which need you to be running in 64-bit mode).  There's not much utility
in having 1GB of your 3GB process address space taken up by a single page.

I'm OK if there are some oddball architectures which support it, but
Linux doesn't.