On Tue, 10 Mar 2020 08:54:05 +0000 Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
> Apologies if anyone gets these twice. I had an email server throttling
> issue yesterday.
>
> On Mon, 24 Feb 2020 13:30:34 +0100
> SeongJae Park <sjpark@xxxxxxxxxx> wrote:
>
> > From: SeongJae Park <sjpark@xxxxxxxxx>
> >
> > This commit introduces a kernel module named DAMON. Note that this
> > commit is implementing only the stub for the module load/unload, basic
> > data structures, and simple manipulation functions of the structures to
> > keep the size of commit small. The core mechanisms of DAMON will be
> > implemented one by one by following commits.
>
> Interesting piece of work. I'm reviewing this partly as an exercise in
> understanding it, but I'll point out minor stuff on the basis I might
> as well whilst I'm here. ;) Note I review bottom up so some comments
> won't make much sense read from the top.
Thanks for review, Jonathan :) I added reply in line below, but agree to your
whole suggestion. Will apply those in next spin.
>
> >
> > Brief Introduction
> > ==================
>
> I'd keep this level of intro for the cover letter / docs. It's not
> particularly useful in commit message it git.
Agreed.
>
> >
[...]
> >
> > +config DAMON
> > + tristate "Data Access Monitor"
> > + depends on MMU
> > + default n
>
> No need to specify a default of n.
Got it.
>
> > + help
> > + Provides data access monitoring.
> > +
> > + DAMON is a kernel module that allows users to monitor the actual
> > + memory access pattern of specific user-space processes. It aims to
> > + be 1) accurate enough to be useful for performance-centric domains,
> > + and 2) sufficiently light-weight so that it can be applied online.
> > +
> > endmenu
[...]
> > +/*
> > + * Construct a damon_region struct
> > + *
> > + * Returns the pointer to the new struct if success, or NULL otherwise
> > + */
> > +static struct damon_region *damon_new_region(struct damon_ctx *ctx,
> > + unsigned long vm_start, unsigned long vm_end)
> > +{
> > + struct damon_region *ret;
>
> I'd give this a different variable name. Expectation in kernel is often
> that ret is simply an magic handle to be passed on. Don't normally expect
> to set elements of it. I'd go long hand and call it region.
Nice point, will change the name to 'region'.
>
> > +
> > + ret = kmalloc(sizeof(struct damon_region), GFP_KERNEL);
>
> sizeof(*ret)
Thanks for catching it! Will apply to other similar cases.
>
> > + if (!ret)
> > + return NULL;
>
> blank line.
Good suggestion.
>
> > + ret->vm_start = vm_start;
> > + ret->vm_end = vm_end;
> > + ret->nr_accesses = 0;
> > + ret->sampling_addr = damon_rand(ctx, vm_start, vm_end);
> > + INIT_LIST_HEAD(&ret->list);
> > +
> > + return ret;
> > +}
> > +
> > +/*
> > + * Add a region between two other regions
> Interestingly even the list.h comments for __list_add call this
> function "insert". No idea why it isn't simply called that..
>
> Perhaps damon_insert_region would be clearer and avoid need
> for comment?
I just wanted to make the name consistent with the 'list.h' file, but your
suggestion sounds better. Will change so.
>
> > + */
> > +static inline void damon_add_region(struct damon_region *r,
> > + struct damon_region *prev, struct damon_region *next)
> > +{
> > + __list_add(&r->list, &prev->list, &next->list);
> > +}
> > +
> > +/*
> > + * Append a region to a task's list of regions
>
> I'd argue the naming is sufficient that the comment adds little.
Yes, will delete it.
>
> > + */
> > +static void damon_add_region_tail(struct damon_region *r, struct damon_task *t)
> > +{
> > + list_add_tail(&r->list, &t->regions_list);
> > +}
> > +
> > +/*
> > + * Delete a region from its list
>
> The list is an implementation detail. I'd not mention that in the comments.
Nice suggestion.
>
> > + */
> > +static void damon_del_region(struct damon_region *r)
> > +{
> > + list_del(&r->list);
> > +}
> > +
> > +/*
> > + * De-allocate a region
>
> Obvious comment - seem rot risk note below.
Agreed.
>
> > + */
> > +static void damon_free_region(struct damon_region *r)
> > +{
> > + kfree(r);
> > +}
> > +
> > +static void damon_destroy_region(struct damon_region *r)
> > +{
> > + damon_del_region(r);
> > + damon_free_region(r);
> > +}
> > +
> > +/*
> > + * Construct a damon_task struct
> > + *
> > + * Returns the pointer to the new struct if success, or NULL otherwise
> > + */
> > +static struct damon_task *damon_new_task(unsigned long pid)
> > +{
> > + struct damon_task *t;
> > +
> > + t = kmalloc(sizeof(struct damon_task), GFP_KERNEL);
>
> sizeof(*t) is probably less error prone if this code is maintained
> in the long run.
Good point, will apply to other cases, either.
>
> > + if (!t)
> > + return NULL;
>
> blank line.
Will add it.
>
> > + t->pid = pid;
> > + INIT_LIST_HEAD(&t->regions_list);
> > +
> > + return t;
> > +}
> > +
> > +/* Returns n-th damon_region of the given task */
> > +struct damon_region *damon_nth_region_of(struct damon_task *t, unsigned int n)
> > +{
> > + struct damon_region *r;
> > + unsigned int i;
> > +
> > + i = 0;
> unsigned int i = 0;
Yes, it must be much better.
>
> > + damon_for_each_region(r, t) {
> > + if (i++ == n)
> > + return r;
> > + }
>
> blank line helps readability a little.
Yes, indeed.
>
> > + return NULL;
> > +}
> > +
> > +static void damon_add_task_tail(struct damon_ctx *ctx, struct damon_task *t)
>
> I'm curious, do we care that it's on the tail? If not I'd look on that as an
> implementation detail and just call this
>
> damon_add_task()
I named it to be consistent with 'damon_add_region[_tail]()' functions, but as
you suggested renaming 'damon_add_region()', it doesn't need to. Will change
the name.
>
> > +{
> > + list_add_tail(&t->list, &ctx->tasks_list);
> > +}
> > +
> > +static void damon_del_task(struct damon_task *t)
> > +{
> > + list_del(&t->list);
> > +}
> > +
> > +static void damon_free_task(struct damon_task *t)
> > +{
> > + struct damon_region *r, *next;
> > +
> > + damon_for_each_region_safe(r, next, t)
> > + damon_free_region(r);
> > + kfree(t);
> > +}
> > +
> > +static void damon_destroy_task(struct damon_task *t)
> > +{
> > + damon_del_task(t);
> > + damon_free_task(t);
> > +}
> > +
> > +/*
> > + * Returns number of monitoring target tasks
>
> As below, kind of obvious so just room for rot.
Agreed.
>
> > + */
> > +static unsigned int nr_damon_tasks(struct damon_ctx *ctx)
> > +{
> > + struct damon_task *t;
> > + unsigned int ret = 0;
> > +
> > + damon_for_each_task(ctx, t)
> > + ret++;
> > + return ret;
> > +}
> > +
> > +/*
> > + * Returns the number of target regions for a given target task
>
> Always a trade off between useful comments and possibility of docs
> rotting. I'd drop this comment certainly.
> The function name is self explanatory.
Agreed!
>
> > + */
> > +static unsigned int nr_damon_regions(struct damon_task *t)
> > +{
> > + struct damon_region *r;
> > + unsigned int ret = 0;
> > +
> > + damon_for_each_region(r, t)
> > + ret++;
>
> Blank line here would help readability a tiny bit.
> Same in other places where we have something followed by a nice
> simple return statement.
Yes, indeed.
>
> > + return ret;
> > +}
> > +
> > +static int __init damon_init(void)
> > +{
> > + pr_info("init\n");
>
> Drop these. They are just noise.
Right, it's just noise, will remove.
Thank you again for kind review, Jonathan!
Thanks,
SeongJae Park
>
> > +
> > + return 0;
> > +}
> > +
> > +static void __exit damon_exit(void)
> > +{
> > + pr_info("exit\n");
> > +}
> > +
> > +module_init(damon_init);
> > +module_exit(damon_exit);
> > +
> > +MODULE_LICENSE("GPL");
> > +MODULE_AUTHOR("SeongJae Park <sjpark@xxxxxxxxx>");
> > +MODULE_DESCRIPTION("DAMON: Data Access MONitor");
>
