Re: [PATCH v8 22/24] mm: Speculative page fault handler return VMA

[Ganesh Mahendran <opensource.ganesh@gmail.com>]

Hi, Laurent

2018-02-16 23:25 GMT+08:00 Laurent Dufour <ldufour@linux.vnet.ibm.com>:
> When the speculative page fault handler is returning VM_RETRY, there is a
> chance that VMA fetched without grabbing the mmap_sem can be reused by the
> legacy page fault handler.  By reusing it, we avoid calling find_vma()
> again. To achieve, that we must ensure that the VMA structure will not be
> freed in our back. This is done by getting the reference on it (get_vma())
> and by assuming that the caller will call the new service
> can_reuse_spf_vma() once it has grabbed the mmap_sem.
>
> can_reuse_spf_vma() is first checking that the VMA is still in the RB tree
> , and then that the VMA's boundaries matched the passed address and release
> the reference on the VMA so that it can be freed if needed.
>
> In the case the VMA is freed, can_reuse_spf_vma() will have returned false
> as the VMA is no more in the RB tree.

when I applied this patch to arm64, I got a crash:

[    6.088296] Unable to handle kernel NULL pointer dereference at
virtual address 00000000
[    6.088307] pgd = ffffff9d67735000
[    6.088313] [00000000] *pgd=00000001795e3003,
*pud=00000001795e3003, *pmd=0000000000000000
[    6.088372] ------------[ cut here ]------------
[    6.088377] Kernel BUG at ffffff9d64f65960 [verbose debug info unavailable]
[    6.088384] Internal error: Oops - BUG: 96000045 [#1] PREEMPT SMP
[    6.088389] BUG: Bad rss-counter state mm:ffffffe8f3861040 idx:0 val:90
[    6.088393] BUG: Bad rss-counter state mm:ffffffe8f3861040 idx:1 val:58
[    6.088398] Modules linked in:
[    6.088408] CPU: 1 PID: 621 Comm: qseecomd Not tainted 4.4.78-perf+ #88
[    6.088413] Hardware name: Qualcomm Technologies, Inc. SDM 636
PM660 + PM660L MTP E7S (DT)
[    6.088419] task: ffffffe8f6208000 ti: ffffffe872a8c000 task.ti:
ffffffe872a8c000
[    6.088432] PC is at __rb_erase_color+0x108/0x240
[    6.088441] LR is at vma_interval_tree_remove+0x244/0x24c
[    6.088447] pc : [<ffffff9d64f65960>] lr : [<ffffff9d64d9c2d8>]
pstate: 604001c5
[    6.088451] sp : ffffffe872a8fa50
[    6.088455] x29: ffffffe872a8fa50 x28: 0000000000000008
[    6.088462] x27: 0000000000000009 x26: 0000000000000000
[    6.088470] x25: ffffffe8f458fb80 x24: 000000768ff87000
[    6.088477] x23: 0000000000000000 x22: 0000000000000000
[    6.088484] x21: ffffff9d64d9be7c x20: ffffffe8f3ff0680
[    6.088492] x19: ffffffe8f212e9b0 x18: 0000000000000074
[    6.088499] x17: 0000000000000007 x16: 000000000000000e
[    6.088507] x15: ffffff9d65c88000 x14: 0000000000000001
[    6.088514] x13: 0000000000192d76 x12: 0000000000989680
[    6.088521] x11: 00000000001fffff x10: ffffff9d661ded1b
[    6.088528] x9 : 0000007691759000 x8 : 0000000007691759
[    6.088535] x7 : 0000000000000000 x6 : ffffffe871ebada8
[    6.088541] x5 : 00000000000000e1 x4 : ffffffe8f212e958
[    6.088548] x3 : 00000000000000e9 x2 : 0000000000000000
[    6.088555] x1 : ffffffe8f212f110 x0 : ffffffe8f212e9b1
[    6.088564]
[    6.088564] PC: 0xffffff9d64f65920:
[    6.088568] 5920  f9000002 aa0103e0 aa1603e1 d63f02a0 aa1603e1
f9400822 f9000662 f9000833
[    6.088590] 5940  1400003b f9400a61 f9400020 370002c0 f9400436
b2400260 f9000a76 f9000433
[    6.088610] 5960  f90002c0 f9400260 f9000020 f9000261 f27ef400
54000100 f9400802 eb13005f
[    6.088630] 5980  54000061 f9000801 14000004 f9000401 14000002
f9000281 aa1303e0 d63f02a0
[    6.088652]
[    6.088652] LR: 0xffffff9d64d9c298:
[    6.088656] c298  f9403083 b4000083 f9400c63 eb03005f 9a832042
f9403883 eb02007f 540000a0
[    6.088676] c2b8  f9003882 f9402c82 927ef442 b5fffd22 b4000080
f0ffffe2 9139f042 94072561
[    6.088695] c2d8  a8c17bfd d65f03c0 a9bf7bfd 910003fd f9400003
d2800000 b40000e3 f9400c65
[    6.088715] c2f8  d1016063 eb0100bf 54000063 aa0303e0 97fffef2
a8c17bfd d65f03c0 a9bf7bfd
[    6.088735]
[    6.088735] SP: 0xffffffe872a8fa10:
[    6.088740] fa10  64d9c2d8 ffffff9d 72a8fa50 ffffffe8 64f65960
ffffff9d 604001c5 00000000
[    6.088759] fa30  71d67d70 ffffffe8 71c281e8 ffffffe8 00000000
00000080 64daa90c ffffff9d
[    6.088779] fa50  72a8fa90 ffffffe8 64d9c2d8 ffffff9d 71ebada8
ffffffe8 f3ff0678 ffffffe8
[    6.088799] fa70  72a8fb80 ffffffe8 00000000 00000000 00000000
00000000 00000001 00000000
[    6.088818]
[    6.088823] Process qseecomd (pid: 621, stack limit = 0xffffffe872a8c028)
[    6.088828] Call trace:
[    6.088834] Exception stack(0xffffffe872a8f860 to 0xffffffe872a8f990)
[    6.088841] f860: ffffffe8f212e9b0 0000008000000000
0000000082b37000 ffffff9d64f65960
[    6.088848] f880: 00000000604001c5 ffffff9d672c8680
ffffff9d672c9c00 ffffff9d672d3ab7
[    6.088855] f8a0: ffffffe872a8f8f0 ffffff9d64db9bfc
0000000000000000 ffffffe8f9402c00
[    6.088861] f8c0: ffffffe872a8c000 0000000000000000
ffffffe872a8f920 ffffff9d64db9bfc
[    6.088867] f8e0: 0000000000000000 ffffffe8f9402b00
ffffffe872a8fa10 ffffff9d64dba568
[    6.088874] f900: ffffffbe61c759c0 ffffffe871d67d70
ffffffe8f9402c00 1de56fb006cba396
[    6.088881] f920: ffffffe8f212e9b1 ffffffe8f212f110
0000000000000000 00000000000000e9
[    6.088888] f940: ffffffe8f212e958 00000000000000e1
ffffffe871ebada8 0000000000000000
[    6.088895] f960: 0000000007691759 0000007691759000
ffffff9d661ded1b 00000000001fffff
[    6.088901] f980: 0000000000989680 0000000000192d76
[    6.088908] [<ffffff9d64f65960>] __rb_erase_color+0x108/0x240
[    6.088915] [<ffffff9d64d9c2d8>] vma_interval_tree_remove+0x244/0x24c
[    6.088924] [<ffffff9d64da4b5c>] __remove_shared_vm_struct+0x74/0x88
[    6.088930] [<ffffff9d64da52b8>] unlink_file_vma+0x40/0x54
[    6.088937] [<ffffff9d64d9f928>] free_pgtables+0xb8/0xfc
[    6.088945] [<ffffff9d64da6b84>] exit_mmap+0x78/0x13c
[    6.088953] [<ffffff9d64c9f5f4>] mmput+0x40/0xe8
[    6.088961] [<ffffff9d64ca5af0>] do_exit+0x3ac/0x8d8
[    6.088966] [<ffffff9d64ca6090>] do_group_exit+0x44/0x9c
[    6.088974] [<ffffff9d64cb10d0>] get_signal+0x4e8/0x524
[    6.088981] [<ffffff9d64c87ea0>] do_signal+0xac/0x93c
[    6.088989] [<ffffff9d64c88a0c>] do_notify_resume+0x18/0x58
[    6.088995] [<ffffff9d64c83038>] work_pending+0x10/0x14
[    6.089003] Code: f9400436 b2400260 f9000a76 f9000433 (f90002c0)
[    6.089009] ---[ end trace 224ce5f97841b6a5 ]---
[    6.110819] Kernel panic - not syncing: Fatal exception

Thanks.

>
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> ---
>  include/linux/mm.h |   5 +-
>  mm/memory.c        | 136 +++++++++++++++++++++++++++++++++--------------------
>  2 files changed, 88 insertions(+), 53 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c383a4e2ceb3..0cd31a37bb3d 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1355,7 +1355,10 @@ extern int handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>                 unsigned int flags);
>  #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>  extern int handle_speculative_fault(struct mm_struct *mm,
> -                                   unsigned long address, unsigned int flags);
> +                                   unsigned long address, unsigned int flags,
> +                                   struct vm_area_struct **vma);
> +extern bool can_reuse_spf_vma(struct vm_area_struct *vma,
> +                             unsigned long address);
>  #endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
>  extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>                             unsigned long address, unsigned int fault_flags,
> diff --git a/mm/memory.c b/mm/memory.c
> index 2ef686405154..1f5ce5ff79af 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4307,13 +4307,22 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  /* This is required by vm_normal_page() */
>  #error "Speculative page fault handler requires __HAVE_ARCH_PTE_SPECIAL"
>  #endif
> -
>  /*
>   * vm_normal_page() adds some processing which should be done while
>   * hodling the mmap_sem.
>   */
> +
> +/*
> + * Tries to handle the page fault in a speculative way, without grabbing the
> + * mmap_sem.
> + * When VM_FAULT_RETRY is returned, the vma pointer is valid and this vma must
> + * be checked later when the mmap_sem has been grabbed by calling
> + * can_reuse_spf_vma().
> + * This is needed as the returned vma is kept in memory until the call to
> + * can_reuse_spf_vma() is made.
> + */
>  int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
> -                            unsigned int flags)
> +                            unsigned int flags, struct vm_area_struct **vma)
>  {
>         struct vm_fault vmf = {
>                 .address = address,
> @@ -4322,7 +4331,6 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>         p4d_t *p4d, p4dval;
>         pud_t pudval;
>         int seq, ret = VM_FAULT_RETRY;
> -       struct vm_area_struct *vma;
>  #ifdef CONFIG_NUMA
>         struct mempolicy *pol;
>  #endif
> @@ -4331,14 +4339,16 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>         flags &= ~(FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_KILLABLE);
>         flags |= FAULT_FLAG_SPECULATIVE;
>
> -       vma = get_vma(mm, address);
> -       if (!vma)
> +       *vma = get_vma(mm, address);
> +       if (!*vma)
>                 return ret;
> +       vmf.vma = *vma;
>
> -       seq = raw_read_seqcount(&vma->vm_sequence); /* rmb <-> seqlock,vma_rb_erase() */
> +       /* rmb <-> seqlock,vma_rb_erase() */
> +       seq = raw_read_seqcount(&vmf.vma->vm_sequence);
>         if (seq & 1) {
> -               trace_spf_vma_changed(_RET_IP_, vma, address);
> -               goto out_put;
> +               trace_spf_vma_changed(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>
>         /*
> @@ -4346,9 +4356,9 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>          * with the VMA.
>          * This include huge page from hugetlbfs.
>          */
> -       if (vma->vm_ops) {
> -               trace_spf_vma_notsup(_RET_IP_, vma, address);
> -               goto out_put;
> +       if (vmf.vma->vm_ops) {
> +               trace_spf_vma_notsup(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>
>         /*
> @@ -4356,18 +4366,18 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>          * because vm_next and vm_prev must be safe. This can't be guaranteed
>          * in the speculative path.
>          */
> -       if (unlikely(!vma->anon_vma)) {
> -               trace_spf_vma_notsup(_RET_IP_, vma, address);
> -               goto out_put;
> +       if (unlikely(!vmf.vma->anon_vma)) {
> +               trace_spf_vma_notsup(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>
> -       vmf.vma_flags = READ_ONCE(vma->vm_flags);
> -       vmf.vma_page_prot = READ_ONCE(vma->vm_page_prot);
> +       vmf.vma_flags = READ_ONCE(vmf.vma->vm_flags);
> +       vmf.vma_page_prot = READ_ONCE(vmf.vma->vm_page_prot);
>
>         /* Can't call userland page fault handler in the speculative path */
>         if (unlikely(vmf.vma_flags & VM_UFFD_MISSING)) {
> -               trace_spf_vma_notsup(_RET_IP_, vma, address);
> -               goto out_put;
> +               trace_spf_vma_notsup(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>
>         if (vmf.vma_flags & VM_GROWSDOWN || vmf.vma_flags & VM_GROWSUP) {
> @@ -4376,48 +4386,39 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>                  * boundaries but we want to trace it as not supported instead
>                  * of changed.
>                  */
> -               trace_spf_vma_notsup(_RET_IP_, vma, address);
> -               goto out_put;
> +               trace_spf_vma_notsup(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>
> -       if (address < READ_ONCE(vma->vm_start)
> -           || READ_ONCE(vma->vm_end) <= address) {
> -               trace_spf_vma_changed(_RET_IP_, vma, address);
> -               goto out_put;
> +       if (address < READ_ONCE(vmf.vma->vm_start)
> +           || READ_ONCE(vmf.vma->vm_end) <= address) {
> +               trace_spf_vma_changed(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>
> -       if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
> +       if (!arch_vma_access_permitted(vmf.vma, flags & FAULT_FLAG_WRITE,
>                                        flags & FAULT_FLAG_INSTRUCTION,
> -                                      flags & FAULT_FLAG_REMOTE)) {
> -               trace_spf_vma_access(_RET_IP_, vma, address);
> -               ret = VM_FAULT_SIGSEGV;
> -               goto out_put;
> -       }
> +                                      flags & FAULT_FLAG_REMOTE))
> +               goto out_segv;
>
>         /* This is one is required to check that the VMA has write access set */
>         if (flags & FAULT_FLAG_WRITE) {
> -               if (unlikely(!(vmf.vma_flags & VM_WRITE))) {
> -                       trace_spf_vma_access(_RET_IP_, vma, address);
> -                       ret = VM_FAULT_SIGSEGV;
> -                       goto out_put;
> -               }
> -       } else if (unlikely(!(vmf.vma_flags & (VM_READ|VM_EXEC|VM_WRITE)))) {
> -               trace_spf_vma_access(_RET_IP_, vma, address);
> -               ret = VM_FAULT_SIGSEGV;
> -               goto out_put;
> -       }
> +               if (unlikely(!(vmf.vma_flags & VM_WRITE)))
> +                       goto out_segv;
> +       } else if (unlikely(!(vmf.vma_flags & (VM_READ|VM_EXEC|VM_WRITE))))
> +               goto out_segv;
>
>  #ifdef CONFIG_NUMA
>         /*
>          * MPOL_INTERLEAVE implies additional check in mpol_misplaced() which
>          * are not compatible with the speculative page fault processing.
>          */
> -       pol = __get_vma_policy(vma, address);
> +       pol = __get_vma_policy(vmf.vma, address);
>         if (!pol)
>                 pol = get_task_policy(current);
>         if (pol && pol->mode == MPOL_INTERLEAVE) {
> -               trace_spf_vma_notsup(_RET_IP_, vma, address);
> -               goto out_put;
> +               trace_spf_vma_notsup(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>  #endif
>
> @@ -4479,9 +4480,8 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>                 vmf.pte = NULL;
>         }
>
> -       vmf.vma = vma;
> -       vmf.pgoff = linear_page_index(vma, address);
> -       vmf.gfp_mask = __get_fault_gfp_mask(vma);
> +       vmf.pgoff = linear_page_index(vmf.vma, address);
> +       vmf.gfp_mask = __get_fault_gfp_mask(vmf.vma);
>         vmf.sequence = seq;
>         vmf.flags = flags;
>
> @@ -4491,16 +4491,22 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>          * We need to re-validate the VMA after checking the bounds, otherwise
>          * we might have a false positive on the bounds.
>          */
> -       if (read_seqcount_retry(&vma->vm_sequence, seq)) {
> -               trace_spf_vma_changed(_RET_IP_, vma, address);
> -               goto out_put;
> +       if (read_seqcount_retry(&vmf.vma->vm_sequence, seq)) {
> +               trace_spf_vma_changed(_RET_IP_, vmf.vma, address);
> +               return ret;
>         }
>
>         mem_cgroup_oom_enable();
>         ret = handle_pte_fault(&vmf);
>         mem_cgroup_oom_disable();
>
> -       put_vma(vma);
> +       /*
> +        * If there is no need to retry, don't return the vma to the caller.
> +        */
> +       if (!(ret & VM_FAULT_RETRY)) {
> +               put_vma(vmf.vma);
> +               *vma = NULL;
> +       }
>
>         /*
>          * The task may have entered a memcg OOM situation but
> @@ -4513,9 +4519,35 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>         return ret;
>
>  out_walk:
> -       trace_spf_vma_notsup(_RET_IP_, vma, address);
> +       trace_spf_vma_notsup(_RET_IP_, vmf.vma, address);
>         local_irq_enable();
> -out_put:
> +       return ret;
> +
> +out_segv:
> +       trace_spf_vma_access(_RET_IP_, vmf.vma, address);
> +       /*
> +        * We don't return VM_FAULT_RETRY so the caller is not expected to
> +        * retrieve the fetched VMA.
> +        */
> +       put_vma(vmf.vma);
> +       *vma = NULL;
> +       return VM_FAULT_SIGSEGV;
> +}
> +
> +/*
> + * This is used to know if the vma fetch in the speculative page fault handler
> + * is still valid when trying the regular fault path while holding the
> + * mmap_sem.
> + * The call to put_vma(vma) must be made after checking the vma's fields, as
> + * the vma may be freed by put_vma(). In such a case it is expected that false
> + * is returned.
> + */
> +bool can_reuse_spf_vma(struct vm_area_struct *vma, unsigned long address)
> +{
> +       bool ret;
> +
> +       ret = !RB_EMPTY_NODE(&vma->vm_rb) &&
> +               vma->vm_start <= address && address < vma->vm_end;
>         put_vma(vma);
>         return ret;
>  }
> --
> 2.7.4
>