Re: [PATCH v3 06/22] mm: Always use page table accessor functions

[David Hildenbrand (Red Hat)]

On 11/26/25 13:27, Lorenzo Stoakes wrote:
> On Wed, Nov 26, 2025 at 01:19:00PM +0100, David Hildenbrand (Red Hat) wrote:
> > On 11/26/25 13:16, David Hildenbrand (Red Hat) wrote:
> > > On 11/26/25 12:09, Ryan Roberts wrote:
> > > > On 13/11/2025 01:45, Samuel Holland wrote:
> > > > > Some platforms need to fix up the values when reading or writing page
> > > > > tables. Because of this, the accessors must always be used; it is not
> > > > > valid to simply dereference a pXX_t pointer.
> > > > >
> > > > > Fix all of the instances of this pattern in generic code, mostly by
> > > > > applying the below coccinelle semantic patch, repeated for each page
> > > > > table level. Some additional fixes were applied manually, mostly to
> > > > > macros where type information is unavailable.
> > > > >
> > > > > In a few places, a `pte_t *` or `pmd_t *` is actually a pointer to a PTE
> > > > > or PMDE value stored on the stack, not a pointer to a page table. In
> > > > > those cases, it is not appropriate to use the accessors, because the
> > > > > value is not globally visible, and any transformation from pXXp_get()
> > > > > has already been applied. Those places are marked by naming the pointer
> > > > > `ptentp` or `pmdvalp`, as opposed to `ptep` or `pmdp`.
> > > > >
> > > > > @@
> > > > > pte_t *P;
> > > > > expression E;
> > > > > expression I;
> > > > > @@
> > > > > - P[I] = E
> > > > > + set_pte(P + I, E)
> > > > >
> > > > > @@
> > > > > pte_t *P;
> > > > > expression E;
> > > > > @@
> > > > > (
> > > > > - WRITE_ONCE(*P, E)
> > > > > + set_pte(P, E)
> > > > > |
> > > > > - *P = E
> > > > > + set_pte(P, E)
> > > > > )
> > > >
> > > > There should absolutely never be any instances of core code directly setting an
> > > > entry at any level. This *must* always go via the arch code helpers. Did you
> > > > find any instances of this? If so, I would consider these bugs and suggest
> > > > sending as a separate bugfix patch. Bad things could happen on arm64 because we
> > > > may need to break a contiguous mapping, which would not happen if the value is
> > > > set directly.
> > > >
> > > > > @@
> > > > > pte_t *P;
> > > > > expression I;
> > > > > @@
> > > > > (
> > > > >      &P[I]
> > > > > |
> > > > > - READ_ONCE(P[I])
> > > > > + ptep_get(P + I)
> > > > > |
> > > > > - P[I]
> > > > > + ptep_get(P + I)
> > > > > )
> > > > >
> > > > > @@
> > > > > pte_t *P;
> > > > > @@
> > > > > (
> > > > > - READ_ONCE(*P)
> > > > > + ptep_get(P)
> > > > > |
> > > > > - *P
> > > > > + ptep_get(P)
> > > > > )
> > > >
> > > > For reading the *PTE*, conversion over to ptep_get() should have already been
> > > > done (I did this a few years back when implementing support for arm64 contiguous
> > > > mappings). If you find any cases where direct dereference or READ_ONCE() is
> > > > being done in generic code for PTE, then that's a bug and should also be sent as
> > > > a separate patch.
> > > >
> > > > FYI, my experience was that Coccinelle didn't find everything when I was
> > > > converting to ptep_get() - although it could have been that my Cochinelle skills
> > > > were not up to scratch! I ended up using an additional method where I did a
> > > > find/replace to convert "pte_t *" to "ptep_handle_t" and declared pte_handle_t
> > > > as a void* which causes a compiler error on dereference. Then in a few key
> > > > places I did a manual case from pte_handle_t to (pte_t *) and compiled allyesconfig.
> > > >
> > > > I'm assuming the above Cocchinelle template was also used for pmd_t, pud_t,
> > > > p4d_t and pgd_t?
> > > >
> > > > > Additionally, the following semantic patch was used to convert PMD and
> > > > > PUD references inside struct vm_fault:
> > > > >
> > > > > @@
> > > > > struct vm_fault vmf;
> > > > > @@
> > > > > (
> > > > > - *vmf.pmd
> > > > > + pmdp_get(vmf.pmd)
> > > > > |
> > > > > - *vmf.pud
> > > > > + pudp_get(vmf.pud)
> > > > > )
> > > > >
> > > > > @@
> > > > > struct vm_fault *vmf;
> > > > > @@
> > > > > (
> > > > > - *vmf->pmd
> > > > > + pmdp_get(vmf->pmd)
> > > > > |
> > > > > - *vmf->pud
> > > > > + pudp_get(vmf->pud)
> > > > > )
> > > > >
> > > > >
> > > > > Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
> > > > > ---
> > > > > This commit covers some of the same changes as an existing series from
> > > > > Anshuman Khandual[1]. Unlike that series, this commit is a purely
> > > > > mechanical conversion to demonstrate the RISC-V changes, so it does not
> > > > > insert local variables to avoid redundant calls to the accessors. A
> > > > > manual conversion like in that series could improve performance.
> > > > >
> > > > > [1]: https://lore.kernel.org/linux-mm/20240917073117.1531207-1-anshuman.khandual@xxxxxxx/
> > > >
> > > > Hi,
> > > >
> > > > I've just come across this patch and wanted to mention that we could also
> > > > benefit from this improved absraction for some features we are looking at for
> > > > arm64. As you mention, Anshuman had a go but hit some roadblocks.
> > > >
> > > > The main issue is that the compiler was unable to optimize away the READ_ONCE()s
> > > > for the case where certain levels of the pgtable are folded. But it can optimize
> > > > the plain C dereferences. There were complaints the the generated code for arm
> > > > (32) and powerpc was significantly impacted due to having many more (redundant)
> > > > loads.
> > >
> > > We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort
> > > this out internally?
> >
> > Just stumbled over the reply from Christope:
> >
> > https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@xxxxxxxxxx
> >
> > And wonder if we could handle that somehow directly in the pgdp_get() etc.
>
> I find that kind of gross to be honest. Isn't the whole point of folding that we
> don't have to think about it...

If we could adjust generic pgdp_get() and friends to not do a 
READ_ONCE() once folded we might not have to think about that in the 
callers.

Just an idea, though, not sure if that would fly the way I envision it.

--
Cheers

David