linux-kernel - Re: [RFC PATCH 05/16] mm/madvise: define and use madvise_behavior struct for madvise_do

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Message-ID: <y2trzz53wb43da2dsdlz44mlyla527zqqslxqgmbgqy753tl62@f7awfhhnfuk2>
Date: Wed, 5 Mar 2025 19:37:28 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: SeongJae Park <sj@...nel.org>
Cc: "Liam R. Howlett" <howlett@...il.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, kernel-team@...a.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 05/16] mm/madvise: define and use madvise_behavior
 struct for madvise_do_behavior()

On Wed, Mar 05, 2025 at 03:56:32PM -0800, SeongJae Park wrote:
> On Wed, 5 Mar 2025 13:40:17 -0800 Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> 
> >  
> > +struct madvise_walk_param {
> > +	int behavior;
> > +	struct anon_vma_name *anon_name;
> > +};
> 
> Only madvise_vma_behavior() will use 'behavior' field.  And only
> madvise_vma_anon_name() will use 'anon_name' field.  But I will have to assume
> both function _might_ use both fields when reading the madvise_walk_vmas()
> function code.  That doesn't make my humble code reading that simple and
> straightforward.
> 
> Also populating and passing a data structure that has data that would not
> really be used seems not very efficient to me.
> 

This is not a new pattern. You can find a lot of examples in kernel
where a struct encapsulates multiple fields and its pointer is passed
around rather than those fields (or subset of them). You can check out
zap_details, vm_fault, fs_parameter. If some fields are mutually
exclusive you can have union in the struct. Also I am not sure what do
you mean by "not efficient" here. Inefficient in what sense? Unused
memory or something else?

> [...]
> > @@ -1666,8 +1674,10 @@ static bool is_memory_populate(int behavior)
> >  }
> >  
> >  static int madvise_do_behavior(struct mm_struct *mm,
> > -		unsigned long start, size_t len_in, int behavior)
> > +		unsigned long start, size_t len_in,
> > +		struct madvise_walk_param *arg)
> >  {
> > +	int behavior = arg->behavior;
> >  	struct blk_plug plug;
> >  	unsigned long end;
> >  	int error;
> > @@ -1681,7 +1691,7 @@ static int madvise_do_behavior(struct mm_struct *mm,
> >  	if (is_memory_populate(behavior))
> >  		error = madvise_populate(mm, start, end, behavior);
> 
> 'arg' is for madvise_walk_vmas() visit function, but we're using it as a
> capsule for passing an information that will be used for madvise_do_behavior().
> This also seems not very straightforward to my humble perspective.

Here we can keep behavior as parameter to madvise_walk_vmas() and define
struct madvise_walk_param inside it with the passed behavior. Anyways
this is a very common pattern in kernel.

> 
> I have no strong opinion and maybe my humble taste is too peculiar.  But, if
> this is not a blocker for tlb flushes batcing, I'd like to suggest keep this
> part as is for now, and revisit for more code cleanup later.  What do you
> think, Shakeel?
> 

Squashing patches 5 to 8 into one is the main request from me. My other
suggestion you can ignore but let's see what other says.