The patch titled
Subject: docs/mm: add more warnings around page table access
has been added to the -mm mm-unstable branch. Its filename is
docs-mm-add-more-warnings-around-page-table-access.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/docs-mm-add-more-warnings-around-page-table-access.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Jann Horn <jannh@xxxxxxxxxx>
Subject: docs/mm: add more warnings around page table access
Date: Mon, 18 Nov 2024 17:47:08 +0100
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.
Link: https://lkml.kernel.org/r/20241118-vma-docs-addition1-onv3-v2-1-c9d5395b72ee@xxxxxxxxxx
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Suggested-by: Matteo Rizzo <matteorizzo@xxxxxxxxxx>
Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Acked-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Cc: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
Cc: SeongJae Park <sj@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/mm/process_addrs.rst | 46 +++++++++++++++++++++------
1 file changed, 36 insertions(+), 10 deletions(-)
--- a/Documentation/mm/process_addrs.rst~docs-mm-add-more-warnings-around-page-table-access
+++ a/Documentation/mm/process_addrs.rst
@@ -339,6 +339,11 @@ When **installing** page table entries,
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 table
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 in
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 in
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
^^^^^^^^^
_
Patches currently in -mm which might be from jannh@xxxxxxxxxx are
docs-mm-add-more-warnings-around-page-table-access.patch
