linux-kernel - Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out vm_map

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Date:   Thu, 19 Jan 2023 20:48:43 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        lstoakes@...il.com, stephen.s.brennan@...cle.com,
        willy@...radead.org, akpm@...ux-foundation.org, hch@...radead.org
Subject: Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out
 vm_map_ram areas

On 01/19/23 at 05:52pm, Baoquan He wrote:
> 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@...hat.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,

Forgot to add details about why there's no race between free_vmap_block()
and vmap_ram_vread() because we have taken vmap_area_lock at the beginning
of vread(). So, except of the missing checking on returned value from
xa_load(), free_vmap_block() either is blocked to wait for vmap_area_lock
before calling unlink_va(), or finishes calling unlink_va() to remove
the vmap from vmap_area_root tree. In both cases, no race happened.

> 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);
> ......
> }
>