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Date:   Thu, 19 Jan 2023 17:52:03 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        lstoakes@gmail.com, stephen.s.brennan@oracle.com,
        willy@infradead.org, akpm@linux-foundation.org, hch@infradead.org
Subject: Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out
 vm_map_ram areas
Message-ID: <Y8kSwyJBe426pj7R@fedora>
References: <20230113031921.64716-1-bhe@redhat.com>
 <20230113031921.64716-4-bhe@redhat.com>
 <Y8U6Gi5d4F8mO0ib@pc636>
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On 01/16/23 at 12:50pm, Uladzislau Rezki wrote:
> On Fri, Jan 13, 2023 at 11:19:17AM +0800, Baoquan He wrote:
> > Currently, vread can read out vmalloc areas which is associated with
> > a vm_struct. While this doesn't work for areas created by vm_map_ram()
> > interface because it doesn't have an associated vm_struct. Then in vread(),
> > these areas are all skipped.
> > 
> > Here, add a new function vmap_ram_vread() to read out vm_map_ram areas.
> > The area created with vmap_ram_vread() interface directly can be handled
> > like the other normal vmap areas with aligned_vread(). While areas
> > which will be further subdivided and managed with vmap_block need
> > carefully read out page-aligned small regions and zero fill holes.
> > 
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > ---
> >  mm/vmalloc.c | 80 +++++++++++++++++++++++++++++++++++++++++++++++-----
> >  1 file changed, 73 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index ab4825050b5c..13875bc41e27 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3544,6 +3544,65 @@ static int aligned_vread(char *buf, char *addr, unsigned long count)
> >  	return copied;
> >  }
> >  
> > +static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags)
> > +{
> > +	char *start;
> > +	struct vmap_block *vb;
> > +	unsigned long offset;
> > +	unsigned int rs, re, n;
> > +
> > +	/*
> > +	 * If it's area created by vm_map_ram() interface directly, but
> > +	 * not further subdividing and delegating management to vmap_block,
> > +	 * handle it here.
> > +	 */
> > +	if (!(flags & VMAP_BLOCK)) {
> > +		aligned_vread(buf, addr, count);
> > +		return;
> > +	}
> > +
> > +	/*
> > +	 * Area is split into regions and tracked with vmap_block, read out
> > +	 * each region and zero fill the hole between regions.
> > +	 */
> > +	vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long)addr));
> > +
> > +	spin_lock(&vb->lock);
> > +	if (bitmap_empty(vb->used_map, VMAP_BBMAP_BITS)) {
> >
> CPU-X invokes free_vmap_block() whereas we take the vb->lock and do
> some manipulations with vb that might be already freed over RCU-core.
> 
> Should we protect it by the rcu_read_lock() also here?

Just go over the vb and vbq code again, seems we don't need the
rcu_read_lock() here. The rcu lock is needed when operating on the
vmap_block_queue->free list. I don't see race between the vb accessing
here and those list adding or removing on vmap_block_queue->free with
rcu. If I miss some race windows between them, please help point out.

However, when I check free_vmap_block(), I do find a risk. As you said,
CPU-x invokes free_vmap_block() and executed xa_erase() to remove the vb
from vmap_blocks tree. Then vread() comes into vmap_ram_vread() and call
xa_load(), it would be null. I should check the returned vb in
free_vmap_block().


static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags)
{
......
if (!(flags & VMAP_BLOCK)) {
                aligned_vread(buf, addr, count);
                return;
        }

        /*
         * Area is split into regions and tracked with vmap_block, read out
         * each region and zero fill the hole between regions.
         */
        vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long)addr));
	if (!vb)    <-- vb need be checked here to avoid accessing erased vb from vmap_blocks tree
		memset(buf, 0, count);
......
}