On 19/09/2024 18:06, Russell King (Oracle) wrote:
On Thu, Sep 19, 2024 at 05:48:58PM +0200, Ryan Roberts wrote:
32-bit arm uses, in some circumstances, an array because each level 1
page table entry is actually two descriptors. It needs to be this way
because each level 2 table pointed to by each level 1 entry has 256
entries, meaning it only occupies 1024 bytes in a 4096 byte page.
In order to cut down on the wastage, treat the level 1 page table as
groups of two entries, which point to two consecutive 1024 byte tables
in the level 2 page.
The level 2 entry isn't suitable for the kernel's use cases (there are
no bits to represent accessed/dirty and other important stuff that the
Linux MM wants) so we maintain the hardware page tables and a separate
set that Linux uses in the same page. Again, the software tables are
consecutive, so from Linux's perspective, the level 2 page tables
have 512 entries in them and occupy one full page.
This is documented in arch/arm/include/asm/pgtable-2level.h
However, what this means is that from the software perspective, the
level 1 page table descriptors are an array of two entries, both of
which need to be setup when creating a level 2 page table, but only
the first one should ever be dereferenced when walking the tables,
otherwise the code that walks the second level of page table entries
will walk off the end of the software table into the actual hardware
descriptors.
I've no idea what the idea is behind introducing pgd_get() and what
it's semantics are, so I can't comment further.
The helper is intended to read the value of the entry pointed to by the passed
in pointer. And it shoiuld be read in a "single copy atomic" manner, meaning no
tearing. Further, the PTL is expected to be held when calling the getter. If the
HW can write to the entry such that its racing with the lock holder (i.e. HW
update of access/dirty) then READ_ONCE() should be suitable for most
architectures. If there is no possibility of racing (because HW doesn't write to
the entry), then a simple dereference would be sufficient, I think (which is
what the core code was already doing in most cases).
The core code should be making no access to the PGD entries on 32-bit
ARM since the PGD level does not exist. Writes are done at PMD level
in arch code. Reads are done by core code at PMD level.
It feels to me like pgd_get() just doesn't fit the model to which 32-bit
ARM was designed to use decades ago, so I want full details about what
pgd_get() is going to be used for and how it is going to be used,
because I feel completely in the dark over this new development. I fear
that someone hasn't understood the Linux page table model if they're
wanting to access stuff at levels that effectively "aren't implemented"
in the architecture specific kernel model of the page tables.
This change isn't as big and scary as I think you fear. The core-mm today
dereferences pgd pointers (and p4d, pud, pmd pointers) directly in its code. See
follow_pfnmap_start(), gup_fast_pgd_leaf(), and many other sites. These changes
aim to abstract those dereferences into an inline function that the architecture
can override and implement if it so wishes.
The core-mm implements default versions of these helper functions which do
READ_ONCE(), but does not currently use them consistently.
