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

[Qi Zheng]

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@xxxxxxxxxx>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>

Acked-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

> +
> +* 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@xxxxxxxxxxxxx/

Thanks!

>