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

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Message-ID: <21195425-53d9-4007-a020-8106f94158dc@bytedance.com>
Date: Tue, 19 Nov 2024 14:53:52 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Jann Horn <jannh@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Jonathan Corbet <corbet@....net>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "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>, Qi Zheng <zhengqi.arch@...edance.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 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/

Thanks!

>