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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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@google.com>
> > Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > Signed-off-by: Jann Horn <jannh@google.com>
>
> Acked-by: Qi Zheng <zhengqi.arch@bytedance.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!
>
> >