Hello Dan
On Thu, Nov 21, 2013 at 3:47 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
> Since zswap_rb_erase was added to the final (when refcount == 0)
> zswap_put_entry, there is no need to call zswap_rb_erase before
> calling zswap_put_entry.
>
> Signed-off-by: Dan Streetman <ddstreet@xxxxxxxx>
> ---
> mm/zswap.c | 5 -----
> 1 file changed, 5 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index e154f1e..f4fbbd5 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -711,8 +711,6 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> ret = zswap_rb_insert(&tree->rbroot, entry, &dupentry);
> if (ret == -EEXIST) {
> zswap_duplicate_entry++;
> - /* remove from rbtree */
> - zswap_rb_erase(&tree->rbroot, dupentry);
> zswap_entry_put(tree, dupentry);
> }
> } while (ret == -EEXIST);
If remove zswap_rb_erase, it would loop until free this dupentry. This
would cause 2 proplems:
1. zswap_duplicate_entry counter is not correct
2. trigger BUG_ON in zswap_entry_put when this dupentry is being writeback,
because zswap_writeback_entry will call zswap_entry_put either.
So, I don't think it is a good idea to remove zswap_rb_erase call.
> @@ -787,9 +785,6 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
> return;
> }
>
> - /* remove from rbtree */
> - zswap_rb_erase(&tree->rbroot, entry);
> -
> /* drop the initial reference from entry creation */
> zswap_entry_put(tree, entry);
I think it is better not to remove the zswap_rb_erase call.
>From frontswap interface view, if invalidate is called, the page(and
entry) should never visible to upper.
If remove the zswap_rb_erase call, it is not fit this semantic.
Consider the following scenario:
1. thread 0: entry A is being writeback
2. thread 1: invalidate entry A, as refcount != 0, it will still exist
on rbtree.
3. thread 1: reuse entry A 's swp_entry_t, do a frontswap_store
it will conflict with the entry A on the rbtree, it is not a
normal duplicate store.
If we place the zswap_rb_erase call in zswap_frontswap_invalidate_page,
we can avoid the above scenario.
So, I don't think it is a good idea to remove zswap_rb_erase call.
Regards,
> --
> 1.8.3.1
>
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