linux-kernel - Re: [PATCH v2 7/9] mm: vmalloc: Support multiple nodes in vread

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Message-ID: <ZP9ZdRc4FDSH2ej4@pc636>
Date:   Mon, 11 Sep 2023 20:16:21 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v2 7/9] mm: vmalloc: Support multiple nodes in vread_iter

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@...il.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@...hat.com>
>
Applied.

Thank you for looking at it!

--
Uladzislau Rezki