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

From: Lorenzo Stoakes
Date: Mon Nov 18 2024 - 12:37:08 EST


On Mon, Nov 18, 2024 at 05:47:08PM +0100, 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>

Nice, LGTM, thanks for this!

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
> Changes in v2:
> - improved based on feedback from Lorenzo
> - Link to v1: https://lore.kernel.org/r/20241114-vma-docs-addition1-onv3-v1-1-ff177a0a2994@xxxxxxxxxx
> ---
> Documentation/mm/process_addrs.rst | 46 +++++++++++++++++++++++++++++---------
> 1 file changed, 36 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst
> index 1bf7ad010fc063d003bb857bb3b695a3eafa0b55..1d416658d7f59ec595bd51018f42eec606f7e272 100644
> --- a/Documentation/mm/process_addrs.rst
> +++ b/Documentation/mm/process_addrs.rst
> @@ -339,6 +339,11 @@ When **installing** page table entries, the mmap or VMA lock must be held to
> keep the VMA stable. We explore why this is in the page table locking details
> section below.
>
> +.. warning:: Page tables are normally only traversed in regions covered by VMAs.
> + If you want to traverse page tables in areas that might not be
> + covered by VMAs, heavier locking is required.
> + See :c:func:`!walk_page_range_novma` for details.
> +
> **Freeing** page tables is an entirely internal memory management operation and
> has special requirements (see the page freeing section below for more details).
>
> @@ -450,6 +455,9 @@ the time of writing of this document.
> Locking Implementation Details
> ------------------------------
>
> +.. warning:: Locking rules for PTE-level page tables are very different from
> + locking rules for page tables at other levels.
> +
> Page table locking details
> --------------------------
>
> @@ -470,8 +478,12 @@ additional locks dedicated to page tables:
> These locks represent the minimum required to interact with each page table
> level, but there are further requirements.
>
> -Importantly, note that on a **traversal** of page tables, no such locks are
> -taken. Whether care is taken on reading the page table entries depends on the
> +Importantly, note that on a **traversal** of page tables, sometimes no such
> +locks are taken. However, at the PTE level, at least concurrent page table
> +deletion must be prevented (using RCU) and the page table must be mapped into
> +high memory, see below.
> +
> +Whether care is taken on reading the page table entries depends on the
> architecture, see the section on atomicity below.
>
> Locking rules
> @@ -489,12 +501,6 @@ We establish basic locking rules when interacting with page tables:
> the warning below).
> * As mentioned previously, zapping can be performed while simply keeping the VMA
> stable, that is holding any one of the mmap, VMA or rmap locks.
> -* Special care is required for PTEs, as on 32-bit architectures these must be
> - mapped into high memory and additionally, careful consideration must be
> - applied to racing with THP, migration or other concurrent kernel operations
> - that might steal the entire PTE table from under us. All this is handled by
> - :c:func:`!pte_offset_map_lock` (see the section on page table installation
> - below for more details).
>
> .. warning:: Populating previously empty entries is dangerous as, when unmapping
> VMAs, :c:func:`!vms_clear_ptes` has a window of time between
> @@ -509,8 +515,28 @@ We establish basic locking rules when interacting with page tables:
> There are additional rules applicable when moving page tables, which we discuss
> in the section on this topic below.
>
> -.. note:: Interestingly, :c:func:`!pte_offset_map_lock` holds an RCU read lock
> - while the PTE page table lock is held.
> +PTE-level page tables are different from page tables at other levels, and there
> +are extra requirements for accessing them:
> +
> +* 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.
> +
> +To access PTE-level page tables, a helper like :c:func:`!pte_offset_map_lock` or
> +:c:func:`!pte_offset_map` can be used depending on stability requirements.
> +These map the page table into kernel memory if required, take the RCU lock, and
> +depending on variant, may also look up or acquire the PTE lock.
> +See the comment on :c:func:`!__pte_offset_map_lock`.
>
> Atomicity
> ^^^^^^^^^
>
> ---
> base-commit: 1e96a63d3022403e06cdda0213c7849b05973cd5
> change-id: 20241114-vma-docs-addition1-onv3-32df4e6dffcf
>
> --
> Jann Horn <jannh@xxxxxxxxxx>
>