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SubjectRe: [PATCH v8] powerpc/mm: Only read faulting instruction when necessary in do_page_fault()
On Wed, 23 May 2018 09:01:19 +0200 (CEST)
Christophe Leroy <christophe.leroy@c-s.fr> wrote:

> Commit a7a9dcd882a67 ("powerpc: Avoid taking a data miss on every
> userspace instruction miss") has shown that limiting the read of
> faulting instruction to likely cases improves performance.
>
> This patch goes further into this direction by limiting the read
> of the faulting instruction to the only cases where it is likely
> needed.
>
> On an MPC885, with the same benchmark app as in the commit referred
> above, we see a reduction of about 3900 dTLB misses (approx 3%):
>
> Before the patch:
> Performance counter stats for './fault 500' (10 runs):
>
> 683033312 cpu-cycles ( +- 0.03% )
> 134538 dTLB-load-misses ( +- 0.03% )
> 46099 iTLB-load-misses ( +- 0.02% )
> 19681 faults ( +- 0.02% )
>
> 5.389747878 seconds time elapsed ( +- 0.06% )
>
> With the patch:
>
> Performance counter stats for './fault 500' (10 runs):
>
> 682112862 cpu-cycles ( +- 0.03% )
> 130619 dTLB-load-misses ( +- 0.03% )
> 46073 iTLB-load-misses ( +- 0.05% )
> 19681 faults ( +- 0.01% )
>
> 5.381342641 seconds time elapsed ( +- 0.07% )
>
> The proper work of the huge stack expansion was tested with the
> following app:
>
> int main(int argc, char **argv)
> {
> char buf[1024 * 1025];
>
> sprintf(buf, "Hello world !\n");
> printf(buf);
>
> exit(0);
> }
>
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> ---
> v8: Back to a single patch as it now makes no sense to split the first part in two. The third patch has no
> dependencies with the ones before, so it will be resend independantly. As suggested by Nicholas, the
> patch now does the get_user() stuff inside bad_stack_expansion(), that's a mid way between v5 and v7.
>
> v7: Following comment from Nicholas on v6 on possibility of the page getting removed from the pagetables
> between the fault and the read, I have reworked the patch in order to do the get_user() in
> __do_page_fault() directly in order to reduce complexity compared to version v5
>
> v6: Rebased on latest powerpc/merge branch ; Using __get_user_inatomic() instead of get_user() in order
> to move it inside the semaphored area. That removes all the complexity of the patch.
>
> v5: Reworked to fit after Benh do_fault improvement and rebased on top of powerpc/merge (65152902e43fef)
>
> v4: Rebased on top of powerpc/next (f718d426d7e42e) and doing access_ok() verification before __get_user_xxx()
>
> v3: Do a first try with pagefault disabled before releasing the semaphore
>
> v2: Changes 'if (cond1) if (cond2)' by 'if (cond1 && cond2)'
>
> arch/powerpc/mm/fault.c | 63 +++++++++++++++++++++++++++++++++++--------------
> 1 file changed, 45 insertions(+), 18 deletions(-)
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 0c99f9b45e8f..7f9363879f4a 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -66,15 +66,11 @@ static inline bool notify_page_fault(struct pt_regs *regs)
> }
>
> /*
> - * Check whether the instruction at regs->nip is a store using
> + * Check whether the instruction inst is a store using
> * an update addressing form which will update r1.
> */
> -static bool store_updates_sp(struct pt_regs *regs)
> +static bool store_updates_sp(unsigned int inst)
> {
> - unsigned int inst;
> -
> - if (get_user(inst, (unsigned int __user *)regs->nip))
> - return false;
> /* check for 1 in the rA field */
> if (((inst >> 16) & 0x1f) != 1)
> return false;
> @@ -233,9 +229,10 @@ static bool bad_kernel_fault(bool is_exec, unsigned long error_code,
> return is_exec || (address >= TASK_SIZE);
> }
>
> -static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
> - struct vm_area_struct *vma,
> - bool store_update_sp)
> +/* Return value is true if bad (sem. released), false if good, -1 for retry */
> +static int bad_stack_expansion(struct pt_regs *regs, unsigned long address,
> + struct vm_area_struct *vma, unsigned int flags,
> + bool is_retry)
> {
> /*
> * N.B. The POWER/Open ABI allows programs to access up to
> @@ -247,10 +244,15 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
> * expand to 1MB without further checks.
> */
> if (address + 0x100000 < vma->vm_end) {
> + struct mm_struct *mm = current->mm;
> + unsigned int __user *nip = (unsigned int __user *)regs->nip;
> + unsigned int inst;
> /* get user regs even if this fault is in kernel mode */
> struct pt_regs *uregs = current->thread.regs;
> - if (uregs == NULL)
> + if (uregs == NULL) {
> + up_read(&mm->mmap_sem);
> return true;
> + }
>
> /*
> * A user-mode access to an address a long way below
> @@ -264,8 +266,30 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
> * between the last mapped region and the stack will
> * expand the stack rather than segfaulting.
> */
> - if (address + 2048 < uregs->gpr[1] && !store_update_sp)
> - return true;
> + if (address + 2048 >= uregs->gpr[1])
> + return false;
> + if (is_retry)
> + return false;
> +
> + if ((flags & FAULT_FLAG_WRITE) && (flags & FAULT_FLAG_USER) &&
> + access_ok(VERIFY_READ, nip, sizeof(inst))) {
> + int res;
> +
> + pagefault_disable();
> + res = __get_user_inatomic(inst, nip);
> + pagefault_enable();
> + if (res) {
> + up_read(&mm->mmap_sem);
> + res = __get_user(inst, nip);
> + if (!res && store_updates_sp(inst))
> + return -1;
> + return true;
> + }
> + if (store_updates_sp(inst))
> + return false;
> + }
> + up_read(&mm->mmap_sem);

Starting to look pretty good... I think probably I prefer the mmap_sem
drop going into the caller so we don't don't drop in the child function.
I thought the retry logic was a little bit complex too, what do you
think of using fault_in_pages_readable and just doing a full retry to
avoid some of this complexity?

> + return true;
> }
> return false;
> }
> @@ -403,7 +427,8 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
> int is_user = user_mode(regs);
> int is_write = page_fault_is_write(error_code);
> int fault, major = 0;
> - bool store_update_sp = false;
> + bool is_retry = false;
> + int is_bad;
>
> if (notify_page_fault(regs))
> return 0;
> @@ -454,9 +479,6 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
> * can result in fault, which will cause a deadlock when called with
> * mmap_sem held
> */
> - if (is_write && is_user)
> - store_update_sp = store_updates_sp(regs);
> -
> if (is_user)
> flags |= FAULT_FLAG_USER;
> if (is_write)
> @@ -503,8 +525,13 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
> return bad_area(regs, address);
>
> /* The stack is being expanded, check if it's valid */
> - if (unlikely(bad_stack_expansion(regs, address, vma, store_update_sp)))
> - return bad_area(regs, address);
> + is_bad = bad_stack_expansion(regs, address, vma, flags, is_retry);
> + if (unlikely(is_bad == -1)) {
> + is_retry = true;
> + goto retry;
> + }
> + if (unlikely(is_bad))
> + return bad_area_nosemaphore(regs, address);

Suggest making the return so that you can do a single unlikely test for
the retry or bad case, and then distinguish the retry in there. Code
generation should be better.

Thanks,
Nick

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