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SubjectRe: [RFC 4/4] speculative pag fault
Hi, Kame. 

On Fri, 18 Dec 2009 09:46:02 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

>
> Lookup vma in lockless style, do page fault, and check mm's version
> after takine page table lock. If racy, mm's version is invalid .
> Then, retry page fault.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> arch/x86/mm/fault.c | 28 +++++++++++++++++++++++++---
> mm/memory.c | 21 ++++++++++++++-------
> 2 files changed, 39 insertions(+), 10 deletions(-)
>
> Index: mmotm-mm-accessor/arch/x86/mm/fault.c
> ===================================================================
> --- mmotm-mm-accessor.orig/arch/x86/mm/fault.c
> +++ mmotm-mm-accessor/arch/x86/mm/fault.c
> @@ -11,6 +11,7 @@
> #include <linux/kprobes.h> /* __kprobes, ... */
> #include <linux/mmiotrace.h> /* kmmio_handler, ... */
> #include <linux/perf_event.h> /* perf_sw_event */
> +#include <linux/hugetlb.h> /* is_vm_hugetlb...*/
>
> #include <asm/traps.h> /* dotraplinkage, ... */
> #include <asm/pgalloc.h> /* pgd_*(), ... */
> @@ -952,6 +953,7 @@ do_page_fault(struct pt_regs *regs, unsi
> struct mm_struct *mm;
> int write;
> int fault;
> + int speculative;
>
> tsk = current;
> mm = tsk->mm;
> @@ -1040,6 +1042,17 @@ do_page_fault(struct pt_regs *regs, unsi
> return;
> }
>
> + if ((error_code & PF_USER) && mm_version_check(mm)) {
> + vma = lookup_vma_cache(mm, address);
> + if (vma && mm_version_check(mm) &&
> + (vma->vm_start <= address) && (address < vma->vm_end)) {
> + speculative = 1;
> + goto found_vma;
> + }
> + if (vma)
> + vma_release(vma);
> + }
> +
> /*
> * When running in the kernel we expect faults to occur only to
> * addresses in user space. All other faults represent errors in
> @@ -1056,6 +1069,8 @@ do_page_fault(struct pt_regs *regs, unsi
> * validate the source. If this is invalid we can skip the address
> * space check, thus avoiding the deadlock:
> */
> +retry_with_lock:
> + speculative = 0;
> if (unlikely(!mm_read_trylock(mm))) {
> if ((error_code & PF_USER) == 0 &&
> !search_exception_tables(regs->ip)) {
> @@ -1073,6 +1088,7 @@ do_page_fault(struct pt_regs *regs, unsi
> }
>
> vma = find_vma(mm, address);
> +found_vma:
> if (unlikely(!vma)) {
> bad_area(regs, error_code, address);
> return;
> @@ -1119,6 +1135,7 @@ good_area:
> */
> fault = handle_mm_fault(mm, vma, address, write ? FAULT_FLAG_WRITE : 0);
>
> +
> if (unlikely(fault & VM_FAULT_ERROR)) {
> mm_fault_error(regs, error_code, address, fault);
> return;
> @@ -1128,13 +1145,18 @@ good_area:
> tsk->maj_flt++;
> perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, 0,
> regs, address);
> - } else {
> + } else if (!speculative || mm_version_check(mm)) {
> tsk->min_flt++;
> perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, 0,
> regs, address);
> + } else {
> + vma_release(vma);
> + goto retry_with_lock;
> }
>
> check_v8086_mode(regs, address, tsk);
> -
> - mm_read_unlock(mm);
> + if (!speculative)
> + mm_read_unlock(mm);
> + else
> + vma_release(vma);
> }
> Index: mmotm-mm-accessor/mm/memory.c
> ===================================================================
> --- mmotm-mm-accessor.orig/mm/memory.c
> +++ mmotm-mm-accessor/mm/memory.c
> @@ -121,6 +121,13 @@ static int __init init_zero_pfn(void)
> }
> core_initcall(init_zero_pfn);
>
> +static bool test_valid_pte(struct mm_struct *mm, pte_t pte, pte_t orig)
> +{
> + if (likely(mm_version_check(mm) && pte_same(pte, orig)))
> + return true;
> + return false;
> +}
> +
> /*
> * If a p?d_bad entry is found while walking page tables, report
> * the error, before resetting entry to p?d_none. Usually (but
> @@ -2044,7 +2051,7 @@ static int do_wp_page(struct mm_struct *
> lock_page(old_page);
> page_table = pte_offset_map_lock(mm, pmd, address,
> &ptl);
> - if (!pte_same(*page_table, orig_pte)) {
> + if (!test_valid_pte(mm, *page_table, orig_pte)) {
> unlock_page(old_page);
> page_cache_release(old_page);
> goto unlock;
> @@ -2105,7 +2112,7 @@ static int do_wp_page(struct mm_struct *
> */
> page_table = pte_offset_map_lock(mm, pmd, address,
> &ptl);
> - if (!pte_same(*page_table, orig_pte)) {
> + if (!test_valid_pte(mm, *page_table, orig_pte)) {
> unlock_page(old_page);
> page_cache_release(old_page);
> goto unlock;
> @@ -2169,7 +2176,7 @@ gotten:
> * Re-check the pte - we dropped the lock
> */
> page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
> - if (likely(pte_same(*page_table, orig_pte))) {
> + if (test_valid_pte(mm, *page_table, orig_pte)) {
> if (old_page) {
> if (!PageAnon(old_page)) {
> dec_mm_counter(mm, file_rss);
> @@ -2555,7 +2562,7 @@ static int do_swap_page(struct mm_struct
> * while we released the pte lock.
> */
> page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
> - if (likely(pte_same(*page_table, orig_pte)))
> + if (pte_same(*page_table, orig_pte))
> ret = VM_FAULT_OOM;
> delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
> goto unlock;
> @@ -2588,7 +2595,7 @@ static int do_swap_page(struct mm_struct
> * Back out if somebody else already faulted in this pte.
> */
> page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
> - if (unlikely(!pte_same(*page_table, orig_pte)))
> + if (unlikely(!test_valid_pte(mm, *page_table, orig_pte)))
> goto out_nomap;
>
> if (unlikely(!PageUptodate(page))) {
> @@ -2844,7 +2851,7 @@ static int __do_fault(struct mm_struct *
> * handle that later.
> */
> /* Only go through if we didn't race with anybody else... */
> - if (likely(pte_same(*page_table, orig_pte))) {
> + if (likely(test_valid_pte(mm, *page_table, orig_pte))) {
> flush_icache_page(vma, page);
> entry = mk_pte(page, vma->vm_page_prot);
> if (flags & FAULT_FLAG_WRITE)
> @@ -2991,7 +2998,7 @@ static inline int handle_pte_fault(struc
>
> ptl = pte_lockptr(mm, pmd);
> spin_lock(ptl);
> - if (unlikely(!pte_same(*pte, entry)))
> + if (unlikely(!test_valid_pte(mm, *pte, entry)))
> goto unlock;
> if (flags & FAULT_FLAG_WRITE) {
> if (!pte_write(entry))
>

I looked over the patch series and come up to one scenario.

CPU A CPU 2

"Thread A reads page"

do_page_fault
lookup_vma_cache
vma->cache_access++
"Thread B unmap the vma"

mm_write_lock
down_write(mm->mmap_sem)
mm->version++
do_munmap
wait_vmas_cache_access
wait_event_interruptible
mm_version_check fail
vma_release
wake_up(vma->cache_wait)
unmap_region
mm_write_unlock
mm_read_trylock
find_vma
!vma
bad_area

As above scenario, Apparently, Thread A reads proper page in the vma at that time.
but it would meet the segment fault by speculative page fault.

Sorry that i don't have time to review more detail.
If I miss something, Pz correct me.

I will review more detail sooner or later. :)

--
Kind regards,
Minchan Kim


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