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SubjectRe: [RFC v7 PATCH 4/4] mm: unmap special vmas with regular do_munmap()
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On 8/10/18 3:46 AM, Vlastimil Babka wrote:
> On 08/10/2018 01:36 AM, Yang Shi wrote:
>> Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or
>> have uprobes set, need get done with write mmap_sem held since
>> they may update vm_flags.
>>
>> So, it might be not safe enough to deal with these kind of special
>> mappings with read mmap_sem. Deal with such mappings with regular
>> do_munmap() call.
>>
>> Michal suggested to make this as a separate patch for safer and more
>> bisectable sake.
>>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> ---
>> mm/mmap.c | 24 ++++++++++++++++++++++++
>> 1 file changed, 24 insertions(+)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 2234d5a..06cb83c 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2766,6 +2766,16 @@ static inline void munlock_vmas(struct vm_area_struct *vma,
>> }
>> }
>>
>> +static inline bool can_zap_with_rlock(struct vm_area_struct *vma)
>> +{
>> + if ((vma->vm_file &&
>> + vma_has_uprobes(vma, vma->vm_start, vma->vm_end)) |
> vma_has_uprobes() seems to be rather expensive check with e.g.
> unconditional spinlock. uprobe_munmap() seems to have some precondition
> cheaper checks for e.g. cases when there's no uprobes in the system
> (should be common?).

I think they are common, i.e. checking vm prot since uprobes are
typically installed for VM_EXEC vmas. We could use those checks to save
some cycles.

>
> BTW, uprobe_munmap() touches mm->flags, not vma->flags, so it should be
> evaluated more carefully for being called under mmap sem for reading, as
> having vmas already detached is no guarantee.

We might just leave uprobe vmas to use regular do_munmap? I'm supposed
they should be not very common. And, uprobes just can be installed for
VM_EXEC vma, although there may be large text segments, typically
VM_EXEC vmas are unmapped when process exits, so the latency might be fine.

>
>> + (vma->vm_flags | (VM_HUGETLB | VM_PFNMAP)))
> ^ I think replace '|' with '&' here?

Yes, thanks for catching this.

>
>> + return false;
>> +
>> + return true;
>> +}
>> +
>> /*
>> * Zap pages with read mmap_sem held
>> *
>> @@ -2808,6 +2818,17 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
>> goto out;
>> }
>>
>> + /*
>> + * Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or
>> + * have uprobes set, need get done with write mmap_sem held since
>> + * they may update vm_flags. Deal with such mappings with regular
>> + * do_munmap() call.
>> + */
>> + for (vma = start_vma; vma && vma->vm_start < end; vma = vma->vm_next) {
>> + if (!can_zap_with_rlock(vma))
>> + goto regular_path;
>> + }
>> +
>> /* Handle mlocked vmas */
>> if (mm->locked_vm) {
>> vma = start_vma;
>> @@ -2828,6 +2849,9 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
>>
>> return 0;
>>
>> +regular_path:
> I think it's missing a down_write_* here.

No, the jump is called before downgrade_write.

Thanks,
Yang

>
>> + ret = do_munmap(mm, start, len, uf);
>> +
>> out:
>> up_write(&mm->mmap_sem);
>> return ret;
>>

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