Re: [PATCH v2 7/9] mm: vmalloc: Support multiple nodes in vread_iter

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Sep 11, 2023 at 11:58:13AM +0800, Baoquan He wrote:
> On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
> > Extend the vread_iter() to be able to perform a sequential
> > reading of VAs which are spread among multiple nodes. So a
> > data read over the /dev/kmem correctly reflects a vmalloc
> > memory layout.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  mm/vmalloc.c | 67 +++++++++++++++++++++++++++++++++++++++++-----------
> >  1 file changed, 53 insertions(+), 14 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 4fd4915c532d..968144c16237 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> ......  
> > @@ -4057,19 +4093,15 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
> >  
> >  	remains = count;
> >  
> > -	/* Hooked to node_0 so far. */
> > -	vn = addr_to_node(0);
> > -	spin_lock(&vn->busy.lock);
> 
> This could change the vread behaviour a little bit. Before, once we take
> vmap_area_lock, the vread will read out the content of snapshot at the
> moment. Now, reading out in one node's tree won't disrupt other nodes'
> tree accessing. Not sure if this matters when people need access
> /proc/kcore, e.g dynamic debugging.
>
With one big tree you anyway drop the lock after one cycle of reading.
As far as i see, kcore.c's read granularity is a PAGE_SIZE.

Please correct me if i am wrong.

> 
> And, the reading will be a little slower because each va finding need
> iterate all vmap_nodes[].
> 
Right. It is a bit tough here, because we have multiple nodes which
represent zones(address space), i.e. there is an offset between them,
it means that, reading fully one tree, will not provide a sequential
reading.

>
> Otherwise, the patch itself looks good to me.
> 
> Reviewed-by: Baoquan He <bhe@redhat.com>
>
Applied.

Thank you for looking at it!

--
Uladzislau Rezki