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=0A=
and 
Hi Andrew Morton=EF=BC=9A

> What are the runtime affects of this bug?
It will give a bad addr which is not at the trigger it?
My understanding was that= it is really rare to trigger,
only if the following conditions a= re met:

1. (info->high_limit - info->low_lim= it) < (info->length + info->align_mask)
2. There is a ad= dr space in mm->mm_rb
     ----
 = ;      |
       |
&n= bsp;      |
      gap_end =3D vma_s= tart_gap(vma0_uesd)    -------------------------------- ---------= ----------|
       |       &nb= sp;                     &= nbsp;                    =                     &nbs= p;                     &n= bsp;                     =                      = ;   |
       |       &nbs= p;                     &n= bsp;                     =                      = ;                     &nb= sp;                     &= nbsp;                    =   |
       |        = ;                     &nb= sp;                     &= nbsp;                    =                     &nbs= p;                     &n= bsp;                     =   |
      info->high_limit     &= nbsp;             -------------------|  =                     &nbs= p;                     &n= bsp;                     = |   > info->length
       |  =                     &nbs= p;                     &n= bsp;                     =     |                 &nb= sp;                     &= nbsp;                    =       |  
       |  = ;                     &nb= sp;                     &= nbsp;                    =     |                 &n= bsp;                     =                      = ;       |
       |   &nbs= p;                     &n= bsp;                     =                      = ;   |   < info->length +   info->alig= n_mask     |
    =   info->low_limit               =                      = ;            |         &n= bsp;                     =                      = ;               |
   = ;    |                 &n= bsp;                     =                      = ;           |           &= nbsp;                    =                     &nbs= p;             |
    &nbs= p;gap_start =3D  vma_end_gap(vma1_used)    -----|--= --------------------------------------------|       
       |
    
In the above case,:
2.1 we will get gep_end firstly=EF= =BC=9A
> while (true) {
> /* Visit right subtree if it looks prom= ising */
> gap_start =3D vma->vm_prev ? vm_end_gap(vma->vm_prev) := 0;
>= if (gap_start <=3D high_limit && vma->vm_rb.rb_right= ) {
>= struct vm_area_struct *right =3D
> rb_entry(vma->vm_rb.rb= _right,
> struct vm_area_struct, vm_rb);
> if (right->rb_su= btree_gap >=3D length) {
> vma =3D right;
> continue;
> }<= /div>
> }
>
> check_current:
> /* Check if curren= t node has a suitable gap */
> gap_end =3D vm_start_gap(vma);
= > if (g= ap_end < low_limit)
> return -ENOMEM;
> if (gap_start <=3D h= igh_limit &&
>    gap_end > gap_start && g= ap_end - gap_start >=3D length) {
> gap_end_tmp =3D gap_end - info-&= gt;length;
> gap_end_tmp -=3D (gap_end_tmp - info->align_offset) &am= p; info->align_mask;
> if (gap_end_tmp >=3D gap_start)
> go= to found;
>
> }

2.2 we clip = it with the original high_limit:
but if info->high_limit -&nbs= p;info->low_limit < info->length + i= nfo->align_mask,
we can not promise get a addr= at the desired alignment, it will cause to return a addr in align error.
   
> found:
> /* We found a = suitable gap. Clip it with the original high_limit. */
> if (gap_end >= info->high_limit)
> gap_end =3D info->high_limit;
><= /div>
> found_highest:
> /* Compute highest gap address at the des= ired alignment */
> gap_end -=3D info->length;
> gap_end -=3D (gap_e= nd - info->align_offset) & info->align_mask;
> VM_BU= G_ON(gap_end < info->low_limit);
> VM_BUG_ON(gap_end < gap_start= );
> = return gap_end;


The pa= tch must require: info->high_limit - = gap_start  > info->length + info->align_mask.
So that when <= span style=3D"font-size: 10.5pt; line-height: 1.5; background-color: transp= arent;">gap_end =3D info->high_limit, we can get a right addr at the des= ired alignment by gap_end.

= Thank you very much indeed for asking such nice problems 
so that i can try my best to explain how the patch work.

pls l= et me know if there are any problem in the patch, thxs.

=0A=
lipeifeng@oppo.com
=0A=
 
Fr= om: Andrew Morton=
Date: 2022-04-13 05:22
To:&nbs= p;lipeifeng
Subject: Re: [PATCH] m= m: fix align-error when get_addr in unmapped_area_topdown
=
On Tue, 12 Apr 2022 16:10:14 +0800 lipeifeng@oppo.com wrote:=0A=
 
=0A=
> From: lipeifeng <lipeifeng@oppo.com>
=0A=
>
=0A=
> when we found a suitable gap_end(> info->high_limit), gap_e= nd
=0A=
> must be set to info->high_limit. And we will get the gap_end=0A=
> after computing highest gap address at the desired alignment.=0A=
>
=0A=
> 2096 found:
=0A=
> 2097         if (gap_end = > info->high_limit)
=0A=
> 2098          &= nbsp;      gap_end =3D info->high_limit;
= =0A=
> 2099
=0A=
> 2100 found_highest:
=0A=
> 2101         gap_end -=3D= info->length;
=0A=
> 2102         gap_end -=3D= (gap_end - info->align_offset) & info->align_mask;
=0A=
> 2103
=0A=
> 2104         VM_BUG_ON(ga= p_end < info->low_limit);
=0A=
> 2105         VM_BUG_ON(ga= p_end < gap_start);
=0A=
> 2106         return gap_e= nd;
=0A=
>
=0A=
> so we must promise: info->high_limit - info->low_limit >= =3D
=0A=
> info->length + info->align_mask.
=0A=
> Or in rare cases(info->high_limit - info->low_limit <=0A=
> info->length + info->align_mask) we will get the addr in=0A=
> align-error if found suitable gap_end(> info->high_limit).<= /div>=0A=
>
=0A=
 
=0A=
Thanks.
=0A=
 
=0A=
What are the runtime affects of this bug, and how are you able to=0A=
trigger it?
=0A=
 
=0A=
> --- a/mm/mmap.c
=0A=
> +++ b/mm/mmap.c
=0A=
> @@ -2009,7 +2009,6 @@ static unsigned long unmapped_area_topdown(= struct vm_unmapped_area_info *info)
=0A=
>  if (length < info->length)
=0A=
>  return -ENOMEM;
=0A=
> 
=0A=
> - length =3D info->length;
=0A=
>  /*
=0A=
>  * Adjust search limits by the desired length.
=0A=
>  * See implementation comment at top of unmapped_area().=0A=
> @@ -2021,6 +2020,8 @@ static unsigned long unmapped_area_topdown(= struct vm_unmapped_area_info *info)
=0A=
> 
=0A=
>  if (info->low_limit > high_limit)
=0A=
>  return -ENOMEM;
=0A=
> +
=0A=
> + length =3D info->length;
=0A=
>  low_limit =3D info->low_limit + length;
=0A=
> 
=0A=
>  /* Check highest gap, which does not precede any rbtree no= de */
=0A=
> --
=0A=
> 2.7.4
=0A=
=0A= = ------=_001_NextPart202423771415_=------