On Wed, 2013-10-09 at 12:16 -0500, Seth Jennings wrote:
> On Wed, Oct 09, 2013 at 10:30:22AM -0500, Seth Jennings wrote:
> > In my approach, I was also looking at allowing the zbud pools to use
> > HIGHMEM pages, since the handle is no longer an address. This requires
> > the pages that are being mapped to be kmapped (atomic) which will
> > disable preemption. This isn't an additional overhead since the
> > map/unmap corresponds with a compress/decompress operation at the zswap
> > level which uses per-cpu variables that disable preemption already.
>
> On second though, lets not mess with the HIGHMEM page support for now.
> Turns out it is tricker than I thought since the unbuddied lists are
> linked through the zbud header stored in the page. But we can still
> disable preemption to allow per-cpu tracking of the current mapping and
> avoid a lookup (and races) in zbud_unmap().
This tracking of current mapping could solve another problem I
encountered with new one-radix-tree approach with storage of duplicated
entries.
The problem is in zbud_unmap() API using offset to unmap (if duplicated
entries are overwritten):
- thread 1: zswap_fronstwap_load() of some offset
- zbud_map() maps this offset -> zhdr1
- thread 2: zswap_frontswap_store() stores new data for this offset
- zbud_alloc() allocated new zhdr2 and replaces zhdr1 in radix tree
under this offset
- new compressed data is stored by zswap
- thread 1: tries to zbud_unmap() of this offset, but now the old
zhdr1 is not present in radix tree so unmap will either fail or use
zhdr2 which is wrong
To solve this issue I experimented with unmapping by zbud_mapped_entry
instead of offset (so zbud_unmap() won't search zbud_header in radix
tree at all):
##########################
int zbud_unmap(struct zbud_pool *pool, pgoff_t offset,
struct zbud_mapped_entry *entry)
{
struct zbud_header *zhdr = handle_to_zbud_header((unsigned
long)entry->addr);
VM_BUG_ON((offset != zhdr->first_offset) && (offset !=
zhdr->last_offset));
spin_lock(&pool->lock);
if (put_map_count(zhdr, offset)) {
/* Racing zbud_free() could not free the offset because
* it was still mapped so it is our job to free. */
zbud_header_free(pool, zhdr, offset);
spin_unlock(&pool->lock);
return -EFAULT;
}
put_zbud_page(zhdr);
spin_unlock(&pool->lock);
return 0;
}
##########################
However getting rid of first/last_map_count seems much more simpler!
Best regards,
Krzysztof
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