Re: [PATCH v4 1/4] mm/slub: enable debugging memory wasting of kmalloc

[Vlastimil Babka]

On 9/5/22 04:55, Feng Tang wrote:
> On Sun, Sep 04, 2022 at 06:58:49PM +0800, Hyeonggon Yoo wrote:
>> On Sun, Sep 04, 2022 at 05:42:33PM +0800, Feng Tang wrote:
>> > On Sun, Sep 04, 2022 at 05:03:34PM +0800, Hyeonggon Yoo wrote:
>> > [...]
>> > > > > 
>> > > > > This patch is okay but with patch 4, init_object() initializes redzone/poison area
>> > > > > using s->object_size, and init_kmalloc_object() fixes redzone/poison area using orig_size.
>> > > > > Why not do it in init_object() in the first time?
>> > > > > 
>> > > > > Also, updating redzone/poison area after alloc_single_from_new_slab()
>> > > > > (outside list_lock, after adding slab to list) will introduce races with validation.
>> > > > > 
>> > > > > So I think doing set_orig_size()/init_kmalloc_object() in alloc_debug_processing() would make more sense.
>> > > > 
>> > > > Yes, this makes sense, and in v3, kmalloc redzone/poison setup was
>> > > > done in alloc_debug_processing() (through init_object()). When
>> > > > rebasing to v4, I met the classical problem: how to pass 'orig_size'
>> > > > parameter :)
>> > > > 
>> > > > In latest 'for-next' branch, one call path for alloc_debug_processing()
>> > > > is
>> > > >   ___slab_alloc
>> > > >     get_partial
>> > > >       get_any_partial
>> > > >         get_partial_node
>> > > >           alloc_debug_processing
>> > > > 
>> > > > Adding 'orig_size' paramter to all these function looks horrible, and
>> > > > I couldn't figure out a good way and chosed to put those ops after
>> > > > 'set_track()'
>> > > 
>> > > IMO adding a parameter to them isn't too horrible...
>> > > I don't see better solution than adding a parameter with current implementation.
>> > > (Yeah, the code is quite complicated...)
>> > > 
>> > > It won't affect performance to meaningful degree as most of
>> > > allocations will be served from cpu slab or percpu partial list. 
>> > 
>> > Thanks for the suggestion! I'm fine with it and just afraid other
>> > developers may dislike the extra parameter. 
>> > 
>> > The race condition you mentioned is a valid concern, and I have thought
>> > about it, one way is moving the set_orig_size() after the redzone/poision
>> > setup, and in 'check_object()' we can detect whether the 'orig_size' is
>> > set, and skip that check if it's not set yet. As the manual validate_slab
>> > triggered from sysfs interface is a rare debug activity, I think skipping
>> > one object shouldn't hurt much.
>> 
>> That will require smp_wmb()/smp_rmb() pair to make sure that
>> effects of set_orig_size() to be visible after redzone/poison setup.
> 
> Yes, synchronization is needed here.
> 
>> Isn't it simpler to add a parameter?
> 
> OK, I can go this way in v5 if other developers are fine. thanks

How about get_partial() instantiates an on-stack structure that contains
gfpflags, ret_slab, orig_size and passes pointer to that to all the nested
functions.

Would be similar to "struct alloc_context" in page allocation.
Something like "struct partial_context pc"?

> - Feng