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

From: Ganesh Mahendran
Date: Wed Mar 28 2018 - 23:09:35 EST

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In reply to: Ganesh Mahendran: "Re: [PATCH v8 22/24] mm: Speculative page fault handler return VMA"
Next in thread: Laurent Dufour: "Re: [PATCH v8 22/24] mm: Speculative page fault handler return VMA"
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2018-03-29 10:26 GMT+08:00 Ganesh Mahendran <opensource.ganesh@xxxxxxxxx>:
> Hi, Laurent
>
> 2018-02-16 23:25 GMT+08:00 Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>:
>> 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.

Fixed by below patch:

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index f6838c0..9c61b0e 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -240,18 +240,18 @@

static int __do_page_fault(struct mm_struct *mm, unsigned long addr,
unsigned int mm_flags, unsigned long vm_flags,
- struct task_struct *tsk, struct vm_area_struct *spf_vma)
+ struct task_struct *tsk, struct vm_area_struct **spf_vma)
{
struct vm_area_struct *vma;
int fault;

#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
- if (spf_vma) {
- if (can_reuse_spf_vma(spf_vma, addr))
- vma = spf_vma;
+ if (*spf_vma) {
+ if (can_reuse_spf_vma(*spf_vma, addr))
+ vma = *spf_vma;
else
vma = find_vma(mm, addr);
- spf_vma = NULL;
+ *spf_vma = NULL;
} else
#endif
vma = find_vma(mm, addr);
@@ -393,7 +389,7 @@
#endif
}

- fault = __do_page_fault(mm, addr, mm_flags, vm_flags, tsk, spf_vma);
+ fault = __do_page_fault(mm, addr, mm_flags, vm_flags, tsk, &spf_vma);

/*
* If we need to retry but a fatal signal is pending, handle the
@@ -480,6 +476,11 @@
return 0;

no_context:
+ if (spf_vma) {
+ put_vma(spf_vma);
+ spf_vma = NULL;
+ }
+
__do_kernel_fault(mm, addr, esr, regs);
return 0;
}

>
>>
>> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
>> ---
>> 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
>>

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