On Wed, Sep 25, 2013 at 5:33 PM, Weijie Yang [off-list ref] wrote:

On Wed, Sep 25, 2013 at 4:31 PM, Bob Liu [off-list ref] wrote:

quoted

On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang [off-list ref] wrote:

quoted

I think I find a new issue, for integrity of this mail thread, I reply
to this mail.

It is a concurrence issue either, when duplicate store and reclaim
concurrentlly.

zswap entry x with offset A is already stored in zswap backend.
Consider the following scenario:

thread 0: reclaim entry x (get refcount, but not call zswap_get_swap_cache_page)

thread 1: store new page with the same offset A, alloc a new zswap entry y.
  store finished. shrink_page_list() call __remove_mapping(), and now
it is not in swap_cache

But I don't think swap layer will call zswap with the same offset A.

1. store page of offset A in zswap
2. some time later, pagefault occur, load page data from zswap.
  But notice that zswap entry x is still in zswap because it is not

Sorry I didn't notice that zswap_frontswap_load() doesn't call rb_erase().

frontswap_tmem_exclusive_gets_enabled.
 this page is with PageSwapCache(page) and page_private(page) = entry.val
3. change this page data, and it become dirty
4. some time later again, swap this page on the same offset A.

so, a duplicate store happens.

Then I think we should erase the entry from rbtree in zswap_frontswap_load().
After the page is decompressed and loaded from zswap, still storing
the compressed data in zswap is meanless.

-- 
Regards,
--Bob

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