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

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Date:   Mon, 16 Jan 2023 19:48:12 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Baoquan He <bhe@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, urezki@...il.com,
        stephen.s.brennan@...cle.com, 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 Mon, Jan 16, 2023 at 07:01:33PM +0000, Matthew Wilcox wrote:
> On Fri, Jan 13, 2023 at 11:19:17AM +0800, Baoquan He wrote:
> > +	spin_lock(&vb->lock);
> > +	if (bitmap_empty(vb->used_map, VMAP_BBMAP_BITS)) {
> > +		spin_unlock(&vb->lock);
> > +		memset(buf, 0, count);
> > +		return;
> > +	}
> > +	for_each_set_bitrange(rs, re, vb->used_map, VMAP_BBMAP_BITS) {
> > +		if (!count)
> > +			break;
> > +		start = vmap_block_vaddr(vb->va->va_start, rs);
> > +		while (addr < start) {
> > +			if (count == 0)
> > +				break;
> > +			*buf = '\0';
> > +			buf++;
> > +			addr++;
> > +			count--;
> > +		}
> > +		/*it could start reading from the middle of used region*/
> > +		offset = offset_in_page(addr);
> > +		n = ((re - rs + 1) << PAGE_SHIFT) - offset;
> > +		if (n > count)
> > +			n = count;
> > +		aligned_vread(buf, start+offset, n);
>
> The whole vread() interface is rather suboptimal.  The only user is proc,
> which is trying to copy to userspace.  But the vread() interface copies
> to a kernel address, so kcore has to copy to a bounce buffer.  That makes
> this spinlock work, but the price is that we can't copy to a user address
> in the future.  Ideally, read_kcore() would be kcore_read_iter() and
> we'd pass an iov_iter into vread().  vread() would then need to use a
> mutex rather than a spinlock.
>
> I don't think this needs to be done now, but if someone's looking for
> a project ...

Interesting! I may take a look at this if I get the time :)