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 rpwFbkSI t9hF1s4M2pJJh4xX3R98Jr3F1W9cjLjYYg1p7P1xCcBoSD0SA7+kkskCgMA/iQGX0BYGxLt0PlPYuVFZWqzBQoMkBqr48wGUZlecfuNNbKV8uThkOOlnQNQfEARJO/Ek4EbF0JuNVnX8nCeSqucNY2jKrPLf58T/y66ychTh2v4ZA+HS3V+mOJ59uA53xQ9hfhDnu3W70qDpl8pOYZYkAo4GK8DgiHsHm0rLaQyN59BxCatrCK3745rmmE3tvz6J2yclfPWEWlVtL2gbYUNvOuFTZonWd8toERA5s+vWRtF1lhjDMaBso/kW6fQ3GuoXQijIjUVUVuCua2gTVFwzaIoxezILZTKZrp6iBOxI+kUJm6fHKuVU1fy2fqHB7l7rK4R5KZBJk47FhOYIEUecJkUek6O1fZVO3ryIdxZNYE1l4544/sVPVscdUENvkF3rZMtD/Z6zf38+ZjPR/MtYc7P2dSJYx/2isu5goolh/UhhAFqbOTptV0yQA46VI2MFOSyvMwHbA5tisptyNRJrzaL96i8mroUrzTmP9rNB5TCz85xmNlvGkJe1sEsWvQ6T6j74XTj68V3xqyphZ2pL5uYeVZRLhLMhj7D0KXQ4pXLSJfaulxvNj3h+308FTYd/8lllGHshdvhBBAgxFleGFtMbGCF+jLctaUGZSOtniGf0i+VVYKHQ8LnnDlHQa+GUYTtfO X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > Suggested-by: Lorenzo Stoakes > Signed-off-by: Jann Horn Nice, LGTM, thanks for this! Reviewed-by: Lorenzo Stoakes > --- > 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@google.com > --- > 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 >