linux-kernel - Re: [PATCH v2] docs/mm: add more warnings around page table access

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Message-ID: <b915ff26-e90d-4151-ab2f-607f3c59f501@lucifer.local>
Date: Tue, 19 Nov 2024 07:48:42 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: Jann Horn <jannh@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Alice Ryhl <aliceryhl@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Matthew Wilcox <willy@...radead.org>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Hillf Danton <hdanton@...a.com>, SeongJae Park <sj@...nel.org>,
        Bagas Sanjaya <bagasdotme@...il.com>, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matteo Rizzo <matteorizzo@...gle.com>
Subject: Re: [PATCH v2] docs/mm: add more warnings around page table access

On Tue, Nov 19, 2024 at 02:53:52PM +0800, Qi Zheng wrote:
>
>
> On 2024/11/19 00:47, Jann Horn wrote:
> > Make it clearer that holding the mmap lock in read mode is not enough
> > to traverse page tables, and that just having a stable VMA is not enough
> > to read PTEs.
> >
> > Suggested-by: Matteo Rizzo <matteorizzo@...gle.com>
> > Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Signed-off-by: Jann Horn <jannh@...gle.com>
>
> Acked-by: Qi Zheng <zhengqi.arch@...edance.com>
>
> > +
> > +* On 32-bit architectures, they may be in high memory (meaning they need to be
> > +  mapped into kernel memory to be accessible).
> > +* When empty, they can be unlinked and RCU-freed while holding an mmap lock or
> > +  rmap lock for reading in combination with the PTE and PMD page table locks.
> > +  In particular, this happens in :c:func:`!retract_page_tables` when handling
> > +  :c:macro:`!MADV_COLLAPSE`.
> > +  So accessing PTE-level page tables requires at least holding an RCU read lock;
> > +  but that only suffices for readers that can tolerate racing with concurrent
> > +  page table updates such that an empty PTE is observed (in a page table that
> > +  has actually already been detached and marked for RCU freeing) while another
> > +  new page table has been installed in the same location and filled with
> > +  entries. Writers normally need to take the PTE lock and revalidate that the
> > +  PMD entry still refers to the same PTE-level page table.
> > +
>
> In practice, this also happens in the retract_page_tables(). Maybe can
> add a note about this after my patch[1] is merged. ;)
>
> [1]. https://lore.kernel.org/lkml/e5b321ffc3ebfcc46e53830e917ad246f7d2825f.1731566457.git.zhengqi.arch@bytedance.com/

You could even queue the doc change up there? :>)

I think one really nice thing with having docs in-tree like this is when we
change things that alter the doc's accuracy we can queue them up with the
patch so the doc always stays in sync.

I feel you may have accidentally self-volunteered there ;)

>
> Thanks!
>
> >