linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Date:   Tue, 25 Oct 2022 15:33:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alistair Popple <apopple@...dia.com>
Cc:     Matthew Wilcox <willy@...radead.org>, Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>, x86@...nel.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, aarcange@...hat.com,
        kirill.shutemov@...ux.intel.com, jroedel@...e.de, ubizjak@...il.com
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

On Tue, Oct 25, 2022 at 06:54:10PM +1100, Alistair Popple wrote:

> > First we (locklessly) load the page table entry, then we grab a
> > reference to the folio that contains it (which can fail if the
> > refcount is zero, in that case we bail), then we recheck that the
> > page table entry is still the same, and if it changed in between,
> > we drop the folio reference and bail.
> > This can, again, grab a reference to a folio after it has
> > already been freed and reallocated. The reason why this is
> > fine is that the metadata structure that holds this refcount,
> > `struct folio` is never freed; even when a folio is
> > freed and reallocated, the corresponding `struct folio`
> > stays.
> 
> I'm probably missing something obvious but how is that synchronised
> against memory hotplug? AFAICT if it isn't couldn't the pages be freed
> and memory removed? In that case the above would no longer hold because
> (I think) the metadata structure could have been freed.

Note, this scheme is older than memory hot-plug, so if anybody is to
blame it's the memory hotplug code.

Anyway, since all that is done with IRQs disabled, all the hotplug stuff
needs to do is rcu_synchronize() in order to ensure all active
IRQ-disabled regions are finshed (between ensuring the memory is unused
and taking out the struct page).