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SubjectRe: [PATCH v8] powerpc/mm: Only read faulting instruction when necessary in do_page_fault()
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Le 23/05/2018 à 09:17, Nicholas Piggin a écrit :
> 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.

Yes I can do that. I though it was ok as the drop is already done in
children functions like bad_area(), bad_access(), ...

> 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?

Yes lets try that way, allthough fault_in_pages_readable() is nothing
else than a get_user().
Should we take any precaution to avoid retrying forever or is it just
not worth it ?

>
>> + 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.

Ok. I'll try and come with v9 during this morning.

Thanks,
Christophe

>
> Thanks,
> Nick
>

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