linux-kernel - Re: [PATCH 02/12] mm/pgtable: add PAE safety to __pte_offset

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Message-ID: <40349492-6f33-2a19-4a5-eabbe6b48aca@google.com>
Date:   Thu, 1 Jun 2023 22:35:10 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
cc:     Hugh Dickins <hughd@...gle.com>,
        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>,
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        Suren Baghdasaryan <surenb@...gle.com>,
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        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
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        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
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Subject: Re: [PATCH 02/12] mm/pgtable: add PAE safety to __pte_offset_map()

On Wed, 31 May 2023, Jason Gunthorpe wrote:
> On Sun, May 28, 2023 at 11:16:16PM -0700, Hugh Dickins wrote:
> > There is a faint risk that __pte_offset_map(), on a 32-bit architecture
> > with a 64-bit pmd_t e.g. x86-32 with CONFIG_X86_PAE=y, would succeed on
> > a pmdval assembled from a pmd_low and a pmd_high which never belonged
> > together: their combination not pointing to a page table at all, perhaps
> > not even a valid pfn.  pmdp_get_lockless() is not enough to prevent that.
> > 
> > Guard against that (on such configs) by local_irq_save() blocking TLB
> > flush between present updates, as linux/pgtable.h suggests.  It's only
> > needed around the pmdp_get_lockless() in __pte_offset_map(): a race when
> > __pte_offset_map_lock() repeats the pmdp_get_lockless() after getting the
> > lock, would just send it back to __pte_offset_map() again.
> 
> What about the other places calling pmdp_get_lockless ? It seems like
> this is quietly making it part of the API that the caller must hold
> the IPIs off.

No, I'm making no judgment of other places where pmdp_get_lockless() is
used: examination might show that some need more care, but I'll just
assume that each is taking as much care as it needs.

But here where I'm making changes, I do see that we need this extra care.

> 
> And Jann had a note that this approach used by the lockless functions
> doesn't work anyhow:
> 
> https://lore.kernel.org/linux-mm/CAG48ez3h-mnp9ZFC10v+-BW_8NQvxbwBsMYJFP8JX31o0B17Pg@mail.gmail.com/

Thanks a lot for the link: I don't know why, but I never saw that mail
thread at all before.  I have not fully digested it yet, to be honest:
MADV_DONTNEED, doesn't flush TLB yet, etc - I'll have to get into the
right frame of mind for that.

> 
> Though we never fixed it, AFAIK..

I'm certainly depending very much on pmdp_get_lockless(): and hoping to
find its case is easier to defend than at the ptep_get_lockless() level.

Thanks,
Hugh