Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Mon, Feb 17, 2020 at 05:08:27PM +0100, Uladzislau Rezki wrote:
> Hello, Joel, Paul, Ted. 
> 
> > 
> > Good point!
> > 
> > Now that kfree_rcu() is on its way to being less of a hack deeply
> > entangled into the bowels of RCU, this might be fairly easy to implement.
> > It might well be simply a matter of a function pointer and a kvfree_rcu()
> > API.  Adding Uladzislau Rezki and Joel Fernandez on CC for their thoughts.
> > 
> I think it makes sense. For example i see there is a similar demand in
> the mm/list_lru.c too. As for implementation, it will not be hard, i think. 
> 
> The easiest way is to inject kvfree() support directly into existing kfree_call_rcu()
> logic(probably will need to rename that function), i.e. to free vmalloc() allocations
> only in "emergency path" by just calling kvfree(). So that function in its turn will
> figure out if it is _vmalloc_ address or not and trigger corresponding "free" path.

The other difference between ext4_kvfree_array_rcu() and kfree_rcu()
is that kfree_rcu() is a magic macro which frees a structure, which
has to contain a struct rcu_head.  In this case, I'm freeing a pointer
to set of structures, or in another case, a set of buffer_heads, which
do *not* have an rcu_head structure.

> struct test_kvfree_rcu {
>        unsigned char array[PAGE_SIZE * 2];
>        struct rcu_head rcu;
> };

I suspect I'd still want to use the ext4_kfree_array_rcu(), for a
couple of reasons.  First of all, the array is variably sized.  So we
don't know how big it is.  That could be fixed via something like 

struct test_kvfree_rcu {
       struct rcu_head rcu;
       struct test_s [];
};

... but the other issue is that we have code where we have arrays of
arrays, e.g.:

	struct ext4_group_info ***s_group_info;

which is an array of array of pointers to ext4_group_info.  Trying to
cram in the rcu_head makes the code more complicated --- and also,
resizing file systems is something that happens often, and I don't
want to optimize it by keeping rcu_head structs around all the time.

This is why at least for *this* use case, it's actually better to
allocate temp array just before callig call_rcu(), and if I can't
allocate it due to memory pressure, we'll it's OK to use
synchronize_rcu() followed by kvfree().

Cheers,

						- Ted