Re: [RFC PATCH] mm: introduce alloc hook to apply PTE_CONT

[Zhaoyang Huang <huangzhaoyang@gmail.com>]

On Tue, Nov 23, 2021 at 5:58 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 23 Nov 2021 at 10:13, Huangzhaoyang <huangzhaoyang@gmail.com> wrote:
> >
> > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> >
> > Since there is no PTE_CONT when rodata_full in ARM64, introducing a
> > hook function to apply PTE_CONT on the proper page blocks.
> >
>
> Given the discussion around your previous patch, I would expect a
> meticulous explanation here why it is guaranteed to be safe to
> manipulate the PTE_CONT attribute like this, and how the proposed
> logic is correct for all supported page sizes.
>
> Without using an intermediate invalid mapping for the entire range,
> this is never going to work reliably (this is the break-before-make
> requirement). And given that marking the entire block invalid will
> create intermediate states that are not permitted (a valid PTE_CONT
> mapping and an invalid ~PTE_CONT mapping covering the same VA), the
> only way to apply changes like these is to temporarily switch all CPUs
> to a different translation via TTBR1. And this is not going to happen.
As there is no safe way to modify PTE_CONT on a live mapping, please
forget all previous patches except current one.
>
> Also, you never replied to my question regarding the use case and the
> performance gain.
In our android based system, the multimedia related cases suffers from
small pte granularity mostly which use high order page blocks quite a
lot. The performance gap even be visible.
>
> In summary, NAK to this patch or any of the previous ones regarding
> PTE_CONT. If you do insist on pursuing this further, please provide an
> elaborate and rock solid explanation why your approach is 100% valid
> and correct (for all page sizes). And make sure you include an
> explanation how your changes comply with the architectural
> break-before-make requirements around PTE_CONT attributes.
IMHO, It is safe to modify the page block's pte undering
*arch_alloc/free_pages* as there is no one else aware of it.
Furthermore, I do think tlbflush and barriers are needed for
synchronization. With regards to page sizes issue, I think replacing
the hard code const value to CONT_PTE_XXX could wrap the difference
and make it correct.

- if ((order >= 4) && (addr & ~CONT_PTE_MASK) == 0) {
-     order -= 4;
+ if ((order >= CONT_PTE_SHIFT) && (addr & ~CONT_PTE_MASK) == 0) {
+    order -= CONT_PTE_SHIFT;
    do {
        cont_pte_low_bound = addr & CONT_PTE_MASK;
    __change_memory_common(cont_pte_low_bound,
    (~CONT_PTE_MASK + 1), __pgprot(PTE_CONT), __pgprot(0));
    addr = (u64)page_address(page);
-     page += 4;
+
    page += CONT_PTES;
    order--;
}while (order >= 0);
>
>
>
> > ---
> >  arch/arm64/include/asm/page.h |  5 +++++
> >  arch/arm64/mm/pageattr.c      | 45 +++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 50 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> > index f98c91b..53cdd09 100644
> > --- a/arch/arm64/include/asm/page.h
> > +++ b/arch/arm64/include/asm/page.h
> > @@ -46,6 +46,11 @@ struct page *alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
> >
> >  #include <asm/memory.h>
> >
> > +#define HAVE_ARCH_ALLOC_PAGE
> > +#define HAVE_ARCH_FREE_PAGE
> > +
> > +extern void arch_alloc_page(struct page *page, int order);
> > +extern void arch_free_page(struct page *page, int order);
> >  #endif /* !__ASSEMBLY__ */
> >
> >  #define VM_DATA_DEFAULT_FLAGS  (VM_DATA_FLAGS_TSK_EXEC | VM_MTE_ALLOWED)
> > diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> > index a3bacd7..815a06d 100644
> > --- a/arch/arm64/mm/pageattr.c
> > +++ b/arch/arm64/mm/pageattr.c
> > @@ -239,3 +239,48 @@ bool kernel_page_present(struct page *page)
> >         ptep = pte_offset_kernel(pmdp, addr);
> >         return pte_valid(READ_ONCE(*ptep));
> >  }
> > +
> > +void arch_alloc_page(struct page *page, int order)
> > +{
> > +       unsigned long addr;
> > +       unsigned long cont_pte_low_bound;
> > +
> > +       if (!rodata_full)
> > +               return;
> > +
> > +       addr = (u64)page_address(page);
> > +       if ((order >= 4) && (addr & ~CONT_PTE_MASK) == 0) {
> > +               order -= 4;
> > +               do {
> > +                       cont_pte_low_bound = addr & CONT_PTE_MASK;
> > +                       __change_memory_common(cont_pte_low_bound,
> > +                                       (~CONT_PTE_MASK + 1), __pgprot(PTE_CONT), __pgprot(0));
> > +                       addr = (u64)page_address(page);
> > +                       page += 4;
> > +                       order--;
> > +               }while (order >= 0);
> > +       }
> > +}
> > +
> > +void arch_free_page(struct page *page, int order)
> > +{
> > +       unsigned long addr;
> > +       unsigned long cont_pte_low_bound;
> > +
> > +       if (!rodata_full)
> > +               return;
> > +
> > +       addr = (u64)page_address(page);
> > +       if ((order >= 4) && (addr & ~CONT_PTE_MASK) == 0) {
> > +               order -= 4;
> > +               do {
> > +                       cont_pte_low_bound = addr & CONT_PTE_MASK;
> > +                       __change_memory_common(cont_pte_low_bound,
> > +                                       (~CONT_PTE_MASK + 1), __pgprot(0), __pgprot(PTE_CONT));
> > +                       addr = (u64)page_address(page);
> > +                       page += 4;
> > +                       order--;
> > +               }while (order >= 0);
> > +       }
> > +}
> > +
> > --
> > 1.9.1
> >