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Date:   Mon, 16 Jan 2023 19:01:33 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org, urezki@gmail.com,
        lstoakes@gmail.com, stephen.s.brennan@oracle.com,
        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
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References: <20230113031921.64716-1-bhe@redhat.com>
 <20230113031921.64716-4-bhe@redhat.com>
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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 ...