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Message-Id: <20240627154721.69aea29609984bd5422afc97@linux-foundation.org>
Date: Thu, 27 Jun 2024 15:47:21 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Yu Zhao <yuzhao@...gle.com>
Cc: Muchun Song <muchun.song@...ux.dev>, David Hildenbrand
<david@...hat.com>, Frank van der Linden <fvdl@...gle.com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>, Peter Xu
<peterx@...hat.com>, Yang Shi <yang@...amperecomputing.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v2] mm/hugetlb_vmemmap: fix race with
speculative PFN walkers
On Thu, 27 Jun 2024 16:27:05 -0600 Yu Zhao <yuzhao@...gle.com> wrote:
> While investigating HVO for THPs [1], it turns out that speculative
> PFN walkers like compaction can race with vmemmap modifications, e.g.,
>
> CPU 1 (vmemmap modifier) CPU 2 (speculative PFN walker)
> ------------------------------- ------------------------------
> Allocates an LRU folio page1
> Sees page1
> Frees page1
>
> Allocates a hugeTLB folio page2
> (page1 being a tail of page2)
>
> Updates vmemmap mapping page1
> get_page_unless_zero(page1)
>
> Even though page1->_refcount is zero after HVO, get_page_unless_zero()
> can still try to modify this read-only field, resulting in a crash.
Ah. So we should backport this into earlier kernels, yes?
Are we able to identify a Fixes: for this? Looks difficult.
This seems quite hard to trigger. Do any particular userspace actions
invoke the race?
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