Messages in this thread | | | Date | Thu, 21 Nov 2013 10:52:55 +0800 | Subject | Re: [PATCH] mm/zswap: remove unneeded zswap_rb_erase calls | From | Weijie Yang <> |
| |
Hello Dan
On Thu, Nov 21, 2013 at 3:47 AM, Dan Streetman <ddstreet@ieee.org> 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@ieee.org> > --- > 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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
| |