Messages in this thread | | | Subject | Re: [PATCH v3 5/6] zswap: replace tree in zswap with radix tree in zbud | From | Krzysztof Kozlowski <> | Date | Thu, 10 Oct 2013 09:01:24 +0200 |
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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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