Re: [PATCH] arm64: mm: pass original fault address to handle_mm_fault() in PER_VMA_LOCK block

[Suren Baghdasaryan]

On Wed, May 24, 2023 at 6:38 AM Jisheng Zhang <jszhang@xxxxxxxxxx> wrote:
>
> On Wed, May 24, 2023 at 09:12:38PM +0800, Jisheng Zhang wrote:
> > When reading the arm64's PER_VMA_LOCK support code, I found a bit
> > difference between arm64 and other arch when calling handle_mm_fault()
> > during VMA lock-based page fault handling: the fault address is masked
> > before passing to handle_mm_fault(). This is also different from the
> > usage in mmap_lock-based handling. I think we need to pass the
> > original fault address to handle_mm_fault() as we did in
> > commit 84c5e23edecd ("arm64: mm: Pass original fault address to
> > handle_mm_fault()").

Thanks for noticing. I'm not sure why this masking leaked into my
patch. I don't think I wrote it before 84c5e23edecd was merged in June
2021.
Anyway, your assessment looks correct to me.

> >
> > If we go through the code path further, we can find that the "masked"
> > fault address can cause mismatched fault address between perf sw
> > major/minor page fault sw event and perf page fault sw event:
>
> OOPS, sorry please ignore this one. I pressed ctrl-c to interrupt the
> git send-mail, but it's still sent out ;)
>
> Instead, let's focus on
> https://lore.kernel.org/linux-arm-kernel/20230524131305.2808-1-jszhang@xxxxxxxxxx/T/#u
>
> The two patches are the same, I just added Suren into CC list.
>
> >
> > do_page_fault
> >   perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, ..., addr)   // orig addr
> >   handle_mm_fault
> >     mm_account_fault
> >       perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, ...) // masked addr
> >
> > Fixes: cd7f176aea5f ("arm64/mm: try VMA lock-based page fault handling first")
> > Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> > ---
> >  arch/arm64/mm/fault.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > index cb21ccd7940d..6045a5117ac1 100644
> > --- a/arch/arm64/mm/fault.c
> > +++ b/arch/arm64/mm/fault.c
> > @@ -600,8 +600,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
> >               vma_end_read(vma);
> >               goto lock_mmap;
> >       }
> > -     fault = handle_mm_fault(vma, addr & PAGE_MASK,
> > -                             mm_flags | FAULT_FLAG_VMA_LOCK, regs);
> > +     fault = handle_mm_fault(vma, addr, mm_flags | FAULT_FLAG_VMA_LOCK, regs);
> >       vma_end_read(vma);
> >
> >       if (!(fault & VM_FAULT_RETRY)) {
> > --
> > 2.40.1
> >
> >
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