On 2024/1/27 03:50, Nhat Pham wrote: > On Fri, Jan 26, 2024 at 12:32 AM <chengming.zhou@xxxxxxxxx> wrote: >> >> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> >> >> LRU writeback has race problem with swapoff, as spotted by Yosry[1]: >> >> CPU1 CPU2 >> shrink_memcg_cb swap_off >> list_lru_isolate zswap_invalidate >> zswap_swapoff >> kfree(tree) >> // UAF >> spin_lock(&tree->lock) >> >> The problem is that the entry in lru list can't protect the tree from >> being swapoff and freed, and the entry also can be invalidated and freed >> concurrently after we unlock the lru lock. >> >> We can fix it by moving the swap cache allocation ahead before >> referencing the tree, then check invalidate race with tree lock, >> only after that we can safely deref the entry. Note we couldn't >> deref entry or tree anymore after we unlock the folio, since we >> depend on this to hold on swapoff. >> >> So this patch moves all tree and entry usage to zswap_writeback_entry(), >> we only use the copied swpentry on the stack to allocate swap cache >> and return with folio locked, after which we can reference the tree. >> Then check invalidate race with tree lock, the following things is >> much the same like zswap_load(). >> >> Since we can't deref the entry after zswap_writeback_entry(), we >> can't use zswap_lru_putback() anymore, instead we rotate the entry > > I added list_lru_putback to the list_lru API specifically for this use > case (zswap_lru_putback()). Now that we no longer need it, maybe we > can also remove this as well (assuming no-one else is using this?). > > This can be done in a separate patch though. Right, I can append a patch to remove it since no other users. > >> in the LRU list so concurrent reclaimers have little chance to see it. >> Or it will be deleted from LRU list if writeback success. >> >> Another confusing part to me is the update of memcg nr_zswap_protected >> in zswap_lru_putback(). I'm not sure why it's needed here since >> if we raced with swapin, memcg nr_zswap_protected has already been >> updated in zswap_folio_swapin(). So not include this part for now. >> >> [1] https://lore.kernel.org/all/CAJD7tkasHsRnT_75-TXsEe58V9_OW6m3g6CF7Kmsvz8CKRG_EA@xxxxxxxxxxxxxx/ >> >> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> > > LGTM! This is quite elegant. > Acked-by: Nhat Pham <nphamcs@xxxxxxxxx> > >> --- >> mm/zswap.c | 93 ++++++++++++++++++------------------------------------ >> 1 file changed, 31 insertions(+), 62 deletions(-) >> >> diff --git a/mm/zswap.c b/mm/zswap.c >> index 81cb3790e0dd..fa2bdb7ec1d8 100644 >> --- a/mm/zswap.c >> +++ b/mm/zswap.c >> @@ -277,7 +277,7 @@ static inline struct zswap_tree *swap_zswap_tree(swp_entry_t swp) >> zpool_get_type((p)->zpools[0])) >> >> static int zswap_writeback_entry(struct zswap_entry *entry, >> - struct zswap_tree *tree); >> + swp_entry_t swpentry); >> static int zswap_pool_get(struct zswap_pool *pool); >> static void zswap_pool_put(struct zswap_pool *pool); >> >> @@ -445,27 +445,6 @@ static void zswap_lru_del(struct list_lru *list_lru, struct zswap_entry *entry) >> rcu_read_unlock(); >> } >> >> -static void zswap_lru_putback(struct list_lru *list_lru, >> - struct zswap_entry *entry) >> -{ >> - int nid = entry_to_nid(entry); >> - spinlock_t *lock = &list_lru->node[nid].lock; >> - struct mem_cgroup *memcg; >> - struct lruvec *lruvec; >> - >> - rcu_read_lock(); >> - memcg = mem_cgroup_from_entry(entry); >> - spin_lock(lock); >> - /* we cannot use list_lru_add here, because it increments node's lru count */ >> - list_lru_putback(list_lru, &entry->lru, nid, memcg); >> - spin_unlock(lock); >> - >> - lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry_to_nid(entry))); >> - /* increment the protection area to account for the LRU rotation. */ >> - atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected); >> - rcu_read_unlock(); >> -} >> - >> /********************************* >> * rbtree functions >> **********************************/ >> @@ -860,40 +839,34 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o >> { >> struct zswap_entry *entry = container_of(item, struct zswap_entry, lru); >> bool *encountered_page_in_swapcache = (bool *)arg; >> - struct zswap_tree *tree; >> - pgoff_t swpoffset; >> + swp_entry_t swpentry; >> enum lru_status ret = LRU_REMOVED_RETRY; >> int writeback_result; >> >> + /* >> + * First rotate to the tail of lru list before unlocking lru lock, >> + * so the concurrent reclaimers have little chance to see it. >> + * It will be deleted from the lru list if writeback success. >> + */ >> + list_move_tail(item, &l->list); >> + >> /* >> * Once the lru lock is dropped, the entry might get freed. The >> - * swpoffset is copied to the stack, and entry isn't deref'd again >> + * swpentry is copied to the stack, and entry isn't deref'd again >> * until the entry is verified to still be alive in the tree. >> */ >> - swpoffset = swp_offset(entry->swpentry); >> - tree = swap_zswap_tree(entry->swpentry); >> - list_lru_isolate(l, item); > > nit: IIUC, now that we're no longer removing the entry from the > list_lru, we protect against concurrent shrinking action via this > check inside zswap_writeback_entry() too right: > > if (!folio_was_allocated) { > folio_put(folio); > return -EEXIST; > } > > Maybe update the comment above it too? * Found an existing folio, we raced with load/swapin. We generally * writeback cold folios from zswap, and swapin means the folio just * became hot. Skip this folio and let the caller find another one. So now found an existing folio not only means load/swapin, and also concurrent shrinking action. Yes, this comment needs to be changed a little. Thanks.
