Unable to understand a piece of code

Unable to understand a piece of code

[Amit Kumar]

HI,

mm/slub.c: line 3973
int __kmem_cache_shrink(struct kmem_cache *s)
{
int node;
int i;
struct kmem_cache_node *n;
struct page *page;
struct page *t;
struct list_head discard;
struct list_head promote[SHRINK_PROMOTE_MAX];
unsigned long flags;
int ret = 0;

flush_all(s);
for_each_kmem_cache_node(s, node, n) {

How uninitialized variable node is being used in macro for_each_kmem_cache_node?

node is a local variable with no extern and not initialized.

mm/slab.h: line 490
#define for_each_kmem_cache_node(__s, __node, __n) \
for (__node = 0; __node < nr_node_ids; __node++) \
if ((__n = get_node(__s, __node)))

As we see for loop is based on node.

Thanks in advance.

Regards,
Amit Kumar

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Re: Unable to understand a piece of code

[Tobin C. Harding]

On Mon, May 20, 2019 at 05:37:41AM +0530, Amit Kumar wrote:
> HI,
> 
> mm/slub.c: line 3973
> int __kmem_cache_shrink(struct kmem_cache *s)
> {
> int node;
> int i;
> struct kmem_cache_node *n;
> struct page *page;
> struct page *t;
> struct list_head discard;
> struct list_head promote[SHRINK_PROMOTE_MAX];
> unsigned long flags;
> int ret = 0;
> 
> flush_all(s);
> for_each_kmem_cache_node(s, node, n) {
> 
> How uninitialized variable node is being used in macro for_each_kmem_cache_node?
>
> node is a local variable with no extern and not initialized.
> 
> mm/slab.h: line 490
> #define for_each_kmem_cache_node(__s, __node, __n) \
> for (__node = 0; __node < nr_node_ids; __node++) \

This _is_ the initialization of node.  Macros are kinda funky, there
isn't much more to say about that other than perhaps read up some more
about how macros work in C and read a bunch more macros in the kernel
e.g. include/linux/list.h

Good luck,
Tobin.

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Re: Unable to understand a piece of code

[Amit Kumar]

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On Mon, 20 May 2019, 6:54 am Tobin C. Harding, <me@tobin.cc> wrote:

> On Mon, May 20, 2019 at 05:37:41AM +0530, Amit Kumar wrote:
> > HI,
> >
> > mm/slub.c: line 3973
> > int __kmem_cache_shrink(struct kmem_cache *s)
> > {
> > int node;
> > int i;
> > struct kmem_cache_node *n;
> > struct page *page;
> > struct page *t;
> > struct list_head discard;
> > struct list_head promote[SHRINK_PROMOTE_MAX];
> > unsigned long flags;
> > int ret = 0;
> >
> > flush_all(s);
> > for_each_kmem_cache_node(s, node, n) {
> >
> > How uninitialized variable node is being used in macro
> for_each_kmem_cache_node?
> >
> > node is a local variable with no extern and not initialized.
> >
> > mm/slab.h: line 490
> > #define for_each_kmem_cache_node(__s, __node, __n) \
> > for (__node = 0; __node < nr_node_ids; __node++) \
>
> This _is_ the initialization of node.

First thank you for your reply.
I just did not use my brain that node is initialized using __node inside
for loop.
I know well macros are literally replaced and do not exist anymore after
preprocessing.

Macros are kinda funky, there
> isn't much more to say about that other than perhaps read up some more
> about how macros work in C and read a bunch more macros in the kernel
> e.g. include/linux/list.h
>
> Good luck,
> Tobin.
>

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Re: Unable to understand a piece of code

[Máté Eckl]

On Mon, May 20, 2019 at 05:37:41AM +0530, Amit Kumar wrote:
> HI,
> 
> mm/slub.c: line 3973
> int __kmem_cache_shrink(struct kmem_cache *s)
> {
> int node;
> int i;
> struct kmem_cache_node *n;
> struct page *page;
> struct page *t;
> struct list_head discard;
> struct list_head promote[SHRINK_PROMOTE_MAX];
> unsigned long flags;
> int ret = 0;
> 
> flush_all(s);
> for_each_kmem_cache_node(s, node, n) {
> 
> How uninitialized variable node is being used in macro for_each_kmem_cache_node?
> 
> node is a local variable with no extern and not initialized.
> 
> mm/slab.h: line 490
> #define for_each_kmem_cache_node(__s, __node, __n) \
> for (__node = 0; __node < nr_node_ids; __node++) \
> if ((__n = get_node(__s, __node)))
> 
> As we see for loop is based on node.

Hi,

This sentence is almost correct. The for loop is *not based* on node but *uses*
the node variable. If you check, __node gets initialised to 0 at the beginning of
the for loop. So whatever you use that macro with will be initialised to 0 at
the beginning of the for loop.

Bests,
Máté

> 
> Thanks in advance.
> 
> Regards,
> Amit Kumar

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