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.le...@c-s.fr> --- 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 | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index fcbb34431da2..dc64b8e06477 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c @@ -450,9 +450,6 @@ 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) - get_user(inst, (unsigned int __user *)regs->nip); - if (is_user) flags |= FAULT_FLAG_USER; if (is_write) @@ -498,6 +495,26 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address, if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) return bad_area(regs, address); + if (unlikely(is_write && is_user && address + 0x100000 < vma->vm_end && + !inst)) { + unsigned int __user *nip = (unsigned int __user *)regs->nip; + + if (likely(access_ok(VERIFY_READ, nip, sizeof(inst)))) { + int res; + + pagefault_disable(); + res = __get_user_inatomic(inst, nip); + pagefault_enable(); + if (unlikely(res)) { + up_read(&mm->mmap_sem); + res = __get_user(inst, nip); + if (!res && inst) + goto retry; + return bad_area_nosemaphore(regs, address); + } + } + } + /* The stack is being expanded, check if it's valid */ if (unlikely(bad_stack_expansion(regs, address, vma, inst))) return bad_area(regs, address); -- 2.13.3