Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out vm_map_ram areas

Matthew Wilcox
2023-01-16T19:01:50Z

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 ...