On 10/20/24 18:20, Lorenzo Stoakes wrote:

The existing generic pagewalk logic permits the walking of page tables,
invoking callbacks at individual page table levels via user-provided
mm_walk_ops callbacks.

This is useful for traversing existing page table entries, but precludes
the ability to establish new ones.

Existing mechanism for performing a walk which also installs page table
entries if necessary are heavily duplicated throughout the kernel, each
with semantic differences from one another and largely unavailable for use
elsewhere.

Rather than add yet another implementation, we extend the generic pagewalk
logic to enable the installation of page table entries by adding a new
install_pte() callback in mm_walk_ops. If this is specified, then upon
encountering a missing page table entry, we allocate and install a new one
and continue the traversal.

If a THP huge page is encountered, we make use of existing logic to split
it. Then once we reach the PTE level, we invoke the install_pte() callback
which provides a PTE entry to install. We do not support hugetlb at this
stage.

If this function returns an error, or an allocation fails during the
operation, we abort the operation altogether. It is up to the caller to
deal appropriately with partially populated page table ranges.

If install_pte() is defined, the semantics of pte_entry() change - this
callback is then only invoked if the entry already exists. This is a useful
property, as it allows a caller to handle existing PTEs while installing
new ones where necessary in the specified range.

If install_pte() is not defined, then there is no functional difference to
this patch, so all existing logic will work precisely as it did before.

As we only permit the installation of PTEs where a mapping does not already
exist there is no need for TLB management, however we do invoke
update_mmu_cache() for architectures which require manual maintenance of
mappings for other CPUs.

We explicitly do not allow the existing page walk API to expose this
feature as it is dangerous and intended for internal mm use only. Therefore
we provide a new walk_page_range_mm() function exposed only to
mm/internal.h.

Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Lorenzo Stoakes <redacted>

<snip>

While the guard pages install ptes unconditionally, maybe some install_pte
handler implementation would sometimes want to skip, should ve define an
error code that means its skipped and just continue instead of set_pte_at()?
Or leave it until such handler appears.

BTW, I find it hard to read this condition even before your patch, oh well.
But if I read it correctly, does it mean we're going to split a pmd-mapped
THP if we have a install_pte handler? But is that really necessary if the
pmd splitting results in all ptes populated, and thus the install_pte
handler can't do anything with any pte anyway?

Ditto?