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SubjectRe: [PATCH] mm,madvise: bugfix of madvise systemcall infinite loop under special circumstances.
On Fri 24-11-17 10:27:57, guoxuenan wrote:
> From: chenjie <chenjie6@huawei.com>
>
> The madvise() system call supported a set of "conventional" advice values,
> the MADV_WILLNEED parameter will trigger an infinite loop under direct
> access mode(DAX). In DAX mode, the function madvise_vma() will return
> directly without updating the pointer [prev].
>
> For example:
> Special circumstances:
> 1、init [ start < vam->vm_start < vam->vm_end < end ]
> 2、madvise_vma() using MADV_WILLNEED parameter ;
> madvise_vma() -> madvise_willneed() -> return 0 && without updating [prev]
>
> =======================================================================
> in Function SYSCALL_DEFINE3(madvise,...)
>
> for (;;)
> {
> //[first loop: start = vam->vm_start < vam->vm_end <end ];
> update [start = vma->vm_start | end ]
>
> con0: if (start >= end) //false always;
> goto out;
> tmp = vma->vm_end;
>
> //do not update [prev] and always return 0;
> error = madvise_willneed();
>
> con1: if (error) //false always;
> goto out;
>
> //[ vam->vm_start < start = vam->vm_end <end ]
> update [start = tmp ]
>
> con2: if (start >= end) //false always ;
> goto out;
>
> //because of pointer [prev] did not change,[vma] keep as it was;
> update [ vma = prev->vm_next ]
> }
>
> =======================================================================
> After the first cycle ;it will always keep
> [ vam->vm_start < start = vam->vm_end < end ].
> since Circulation exit conditions (con{0,1,2}) will never meet ,the
> program stuck in infinite loop.

Are you sure? Have you tested this? I might be missing something because
madvise code is a bit of a mess but AFAICS prev pointer (updated or not)
will allow to move advance
if (prev)
vma = prev->vm_next;
else /* madvise_remove dropped mmap_sem */
vma = find_vma(current->mm, start);
note that start is vma->vm_end and find_vma will find a vma which
vma_end > addr

So either I am missing something or this code has actaully never worked
for DAX, XIP which I find rather suspicious.

> Signed-off-by: chenjie <chenjie6@huawei.com>
> Signed-off-by: guoxuenan <guoxuenan@huawei.com>
> ---
> mm/madvise.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 21261ff..c355fee 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -294,6 +294,7 @@ static long madvise_willneed(struct vm_area_struct *vma,
> #endif
>
> if (IS_DAX(file_inode(file))) {
> + *prev = vma;
> /* no bad return value, but ignore advice */
> return 0;
> }
> --
> 2.9.5
>

--
Michal Hocko
SUSE Labs

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