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SubjectRe: [PATCH] mm,madvise: bugfix of madvise systemcall infinite loop under special circumstances.
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Yes , your modification is much better! thanks.

在 2017/11/24 21:08, Michal Hocko 写道:
> On Fri 24-11-17 20:51:29, 郭雪楠 wrote:
>> Sorry,I explained wrong before. But,I've tested using trinity in DAX
>> mode,and I'am sure it has possibility of triggering an soft lockup. I have
>> encountered the problem of endless loop here .
>>
>> I had a little problem here,I correct it .
>> under Initial state :
>> [ start = vam->vm_start < vam->vm_end < end ]
>>
>> When [start = vam->vm_start] the program enters for{;;} loop
>> ,find_vma_prev() will set the pointer vma and the pointer prev (prev =
>> vam->vm_prev ). Normally ,madvise_vma() will always move the pointer prev
>> ,but when use DAX mode , it will never update .
> [...]
>> if (prev) // here prev not NULL,it will always enter this branch ..
>> vma = prev->vm_next;
>> else /* madvise_remove dropped mmap_sem */
>> vma = find_vma(current->mm, start);
>
> You are right! My fault, I managed to confuse myself in the code flow.
> It really looks like this has been broken for more than 10 years since
> fe77ba6f4f97 ("[PATCH] xip: madvice/fadvice: execute in place").
>
> Maybe the following would be more readable and less error prone?
> ---
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 375cf32087e4..a631c414f915 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -276,30 +276,26 @@ static long madvise_willneed(struct vm_area_struct *vma,
> {
> struct file *file = vma->vm_file;
>
> + *prev = vma;
> #ifdef CONFIG_SWAP
> if (!file) {
> - *prev = vma;
> force_swapin_readahead(vma, start, end);
> return 0;
> }
>
> - if (shmem_mapping(file->f_mapping)) {
> - *prev = vma;
> + if (shmem_mapping(file->f_mapping))
> force_shm_swapin_readahead(vma, start, end,
> file->f_mapping);
> return 0;
> - }
> #else
> if (!file)
> return -EBADF;
> #endif
>
> - if (IS_DAX(file_inode(file))) {
> + if (IS_DAX(file_inode(file)))
> /* no bad return value, but ignore advice */
> return 0;
> - }
>
> - *prev = vma;
> start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> if (end > vma->vm_end)
> end = vma->vm_end;
>

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