Re: [PATCH] mm,madvise: bugfix of madvise systemcall infinite loop under special circumstances.
From: guoxuenan
Date: Mon Nov 27 2017 - 07:28:42 EST
Of course! Thank you, you save my poor english :).
å 2017/11/27 19:58, Michal Hocko åé:
On Mon 27-11-17 19:53:18, guoxuenan wrote:
From: chenjie <chenjie6@xxxxxxxxxx>
The madvise() system call supported a set of "conventional" advice values,
the MADV_WILLNEED parameter has possibility of triggering an infinite loop under
direct access mode(DAX).
Infinite loop situation:
1ãinitial state [ start = vam->vm_start < vam->vm_end < end ].
2ãmadvise_vma() using MADV_WILLNEED parameter;
madvise_vma() -> madvise_willneed() -> return 0 && the value of [prev] is not updated.
In function SYSCALL_DEFINE3(madvise,...)
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 the value of [prev].
=======================================================================
SYSCALL_DEFINE3(madvise,...)
{
[...]
//start = vam->start => prev=vma->prev
vma = find_vma_prev(current->mm, start, &prev);
[...]
for(;;)
{
update [start = vma->vm_start]
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 && vma = prev->vm_next;
since Circulation exit conditions (con{0,1,2}) will never meet ,the
program stuck in infinite loop.
I find your changelog a bit hard to parse. What would you think about
the following:
"
MADVISE_WILLNEED has always been a noop for DAX (formerly XIP) mappings.
Unfortunatelly madvise_willneed doesn't communicate this information
properly to the generic madvise syscall implementation. The calling
converion is quite subtle there. madvise_vma is supposed to either
return an error or update &prev otherwise the main loop will never
advance to the next vma and it will keep looping for ever without a way
to get out of the kernel.
It seems this has been broken since introduced. Nobody has noticed
because nobody seems to be using MADVISE_WILLNEED on these DAX mappings.
Fixes: fe77ba6f4f97 ("[PATCH] xip: madvice/fadvice: execute in place")
Cc: stable
"
Signed-off-by: chenjie <chenjie6@xxxxxxxxxx>
Signed-off-by: guoxuenan <guoxuenan@xxxxxxxxxx>
Other than that
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
---
mm/madvise.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/mm/madvise.c b/mm/madvise.c
index 375cf32..751e97a 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -276,15 +276,14 @@ 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;
force_shm_swapin_readahead(vma, start, end,
file->f_mapping);
return 0;
@@ -299,7 +298,6 @@ static long madvise_willneed(struct vm_area_struct *vma,
return 0;
}
- *prev = vma;
start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
if (end > vma->vm_end)
end = vma->vm_end;
--
2.9.5