[RFC PATCH] mm/vmalloc: make sure to traverse from the beginning when overflow occur

[RFC PATCH] mm/vmalloc: make sure to traverse from the beginning when overflow occur

[Liu Song]

From: Liu Song <liu.song11@zte.com.cn>

If overflow, should ensure that "free_vmap_cache" is set to NULL,
so as to ensure that it can be traversed from the beginning.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
---
 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d8e877365f9f..2638a20d36ce 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -441,7 +441,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
 	if (!free_vmap_cache ||
 			size < cached_hole_size ||
 			vstart < cached_vstart ||
-			align < cached_align) {
+			align < cached_align || purged) {
 nocache:
 		cached_hole_size = 0;
 		free_vmap_cache = NULL;
-- 
2.20.1

Re: [RFC PATCH] mm/vmalloc: make sure to traverse from the beginning when overflow occur

[Dave Hansen]

On 4/12/20 7:38 AM, Liu Song wrote:
> If overflow, should ensure that "free_vmap_cache" is set to NULL,
> so as to ensure that it can be traversed from the beginning.

This changelog is a bit sparse.

Does this fix a demonstrated problem?  Or was it just discovered via
code review and assumed to help a theoretical problem?

Which tree is it against?  'free_vmap_cache' doesn't show up in Linus's
tree.
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d8e877365f9f..2638a20d36ce 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -441,7 +441,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  	if (!free_vmap_cache ||
>  			size < cached_hole_size ||
>  			vstart < cached_vstart ||
> -			align < cached_align) {
> +			align < cached_align || purged) {
>  nocache:
>  		cached_hole_size = 0;
>  		free_vmap_cache = NULL;

Is 'purged' the right way to do this?

There is a path through the overflow code that also sets purged=0.  Does
'free_vmap_cache' *not* need to get reset in that case?

It also seems more natural to me that the code mucking with the rbtree
would be the one to reset the cache.

Re: [RFC PATCH] mm/vmalloc: make sure to traverse from the beginning when overflow occur

[fishland]

>On 4/12/20 7:38 AM, Liu Song wrote:
>> If overflow, should ensure that "free_vmap_cache" is set to NULL,
>> so as to ensure that it can be traversed from the beginning.
>
>This changelog is a bit sparse.
>
>Does this fix a demonstrated problem?  Or was it just discovered via
>code review and assumed to help a theoretical problem?
>
>Which tree is it against?  'free_vmap_cache' doesn't show up in Linus's
>tree.
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index d8e877365f9f..2638a20d36ce 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -441,7 +441,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>>   if (!free_vmap_cache ||
>>     size < cached_hole_size ||
>>     vstart < cached_vstart ||
>> -   align < cached_align) {
>> +   align < cached_align || purged) {
>>  nocache:
>>    cached_hole_size = 0;
>>    free_vmap_cache = NULL;
>
>Is 'purged' the right way to do this?
>
>There is a path through the overflow code that also sets purged=0.  Does
>'free_vmap_cache' *not* need to get reset in that case?
>
>It also seems more natural to me that the code mucking with the rbtree
>would be the one to reset the cache.

Hi, 
Thanks for your reply, there is indeed a problem with this patch submission.
I neglected the cache clearing process, so I mistakenly thought that the cache
should be cleared after "purged". I will not issue such unconsidered patches
anymore, to avoid unnecessary trouble for everyone. *_^

Thanks

Re: [RFC PATCH] mm/vmalloc: make sure to traverse from the beginning when overflow occur

[Uladzislau Rezki]

On Mon, Apr 13, 2020 at 08:02:49AM -0700, Dave Hansen wrote:
> On 4/12/20 7:38 AM, Liu Song wrote:
> > If overflow, should ensure that "free_vmap_cache" is set to NULL,
> > so as to ensure that it can be traversed from the beginning.
> 
> This changelog is a bit sparse.
> 
> Does this fix a demonstrated problem?  Or was it just discovered via
> code review and assumed to help a theoretical problem?
> 
> Which tree is it against?  'free_vmap_cache' doesn't show up in Linus's
> tree.
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index d8e877365f9f..2638a20d36ce 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -441,7 +441,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
> >  	if (!free_vmap_cache ||
> >  			size < cached_hole_size ||
> >  			vstart < cached_vstart ||
> > -			align < cached_align) {
> > +			align < cached_align || purged) {
> >  nocache:
> >  		cached_hole_size = 0;
> >  		free_vmap_cache = NULL;
> >
We do not have such code since 5.2 kernel or so, nor things like
cached_hole_size, free_vmap_cache and so on. It was totally reworked.

--
Vlad Rezki