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> --- 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>
