On Tue, Oct 24, 2023 at 8:17 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > On Tue, Oct 24, 2023 at 1:33 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote: > > > > From: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx> > > > > Currently, we only have a single global LRU for zswap. This makes it > > impossible to perform worload-specific shrinking - an memcg cannot > > determine which pages in the pool it owns, and often ends up writing > > pages from other memcgs. This issue has been previously observed in > > practice and mitigated by simply disabling memcg-initiated shrinking: > > > > https://lore.kernel.org/all/20230530232435.3097106-1-nphamcs@xxxxxxxxx/T/#u > > > > This patch fully resolves the issue by replacing the global zswap LRU > > with memcg- and NUMA-specific LRUs, and modify the reclaim logic: > > > > a) When a store attempt hits an memcg limit, it now triggers a > > synchronous reclaim attempt that, if successful, allows the new > > hotter page to be accepted by zswap. > > b) If the store attempt instead hits the global zswap limit, it will > > trigger an asynchronous reclaim attempt, in which an memcg is > > selected for reclaim in a round-robin-like fashion. > > > > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx> > > Co-developed-by: Nhat Pham <nphamcs@xxxxxxxxx> > > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx> > > --- > > include/linux/memcontrol.h | 5 + > > mm/swap.h | 3 +- > > mm/swap_state.c | 23 +++-- > > mm/zswap.c | 188 ++++++++++++++++++++++++++----------- > > 4 files changed, 156 insertions(+), 63 deletions(-) > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > > index 6edd3ec4d8d5..c1846e57011b 100644 > > --- a/include/linux/memcontrol.h > > +++ b/include/linux/memcontrol.h > > @@ -1187,6 +1187,11 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page) > > return NULL; > > } > > > > +static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg) > > +{ > > + return NULL; > > +} > > + > > static inline bool folio_memcg_kmem(struct folio *folio) > > { > > return false; > > diff --git a/mm/swap.h b/mm/swap.h > > index 73c332ee4d91..c0dc73e10e91 100644 > > --- a/mm/swap.h > > +++ b/mm/swap.h > > @@ -51,7 +51,8 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, > > struct swap_iocb **plug); > > struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, > > struct mempolicy *mpol, pgoff_t ilx, > > - bool *new_page_allocated); > > + bool *new_page_allocated, > > + bool skip_if_exists); > > struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t flag, > > struct mempolicy *mpol, pgoff_t ilx); > > struct page *swapin_readahead(swp_entry_t entry, gfp_t flag, > > diff --git a/mm/swap_state.c b/mm/swap_state.c > > index 85d9e5806a6a..040639e1c77e 100644 > > --- a/mm/swap_state.c > > +++ b/mm/swap_state.c > > @@ -412,7 +412,8 @@ struct folio *filemap_get_incore_folio(struct address_space *mapping, > > > > struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, > > struct mempolicy *mpol, pgoff_t ilx, > > - bool *new_page_allocated) > > + bool *new_page_allocated, > > + bool skip_if_exists) > > { > > struct swap_info_struct *si; > > struct folio *folio; > > @@ -470,6 +471,16 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, > > if (err != -EEXIST) > > goto fail_put_swap; > > > > + /* Protect against a recursive call to __read_swap_cache_async() > > nit: insert new line before "Protect", see surrounding comments. > > > + * on the same entry waiting forever here because SWAP_HAS_CACHE > > + * is set but the folio is not the swap cache yet. This can > > + * happen today if mem_cgroup_swapin_charge_folio() below > > + * triggers reclaim through zswap, which may call > > + * __read_swap_cache_async() in the writeback path. > > + */ > > + if (skip_if_exists) > > + goto fail_put_swap; > > + > > /* > > * We might race against __delete_from_swap_cache(), and > > * stumble across a swap_map entry whose SWAP_HAS_CACHE > [..] > > +/********************************* > > +* lru functions > > +**********************************/ > > +static bool zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry) > > +{ > > + struct mem_cgroup *memcg = get_mem_cgroup_from_entry(entry); > > + int nid = entry_to_nid(entry); > > + bool added = list_lru_add(list_lru, &entry->lru, nid, memcg); > > + > > + mem_cgroup_put(memcg); > > Still not fond of the get/put pattern but okay.. Actually, Johannes and I took another look to see if we can replace the memcg reference getting with just rcu_read_lock(). It seems there might be a race between zswap LRU manipulation and memcg offlining - not just with the rcu_read_lock() idea, but also with our current implementation! I'll shoot another email with more details later when I'm sure of it one way or another... > > > + return added; > > +} > > + > > +static bool zswap_lru_del(struct list_lru *list_lru, struct zswap_entry *entry) > > +{ > > + struct mem_cgroup *memcg = get_mem_cgroup_from_entry(entry); > > + int nid = entry_to_nid(entry); > > + bool removed = list_lru_del(list_lru, &entry->lru, nid, memcg); > > + > > + mem_cgroup_put(memcg); > > + return removed; > > +} > > + > > /********************************* > > * rbtree functions > > **********************************/ > [..] > > @@ -652,28 +679,37 @@ static int zswap_reclaim_entry(struct zswap_pool *pool) > > */ > > swpoffset = swp_offset(entry->swpentry); > > tree = zswap_trees[swp_type(entry->swpentry)]; > > - spin_unlock(&pool->lru_lock); > > + list_lru_isolate(l, item); > > + /* > > + * It's safe to drop the lock here because we return either > > + * LRU_REMOVED_RETRY or LRU_RETRY. > > + */ > > + spin_unlock(lock); > > > > /* Check for invalidate() race */ > > spin_lock(&tree->lock); > > - if (entry != zswap_rb_search(&tree->rbroot, swpoffset)) { > > - ret = -EAGAIN; > > + if (entry != zswap_rb_search(&tree->rbroot, swpoffset)) > > goto unlock; > > - } > > + > > /* Hold a reference to prevent a free during writeback */ > > zswap_entry_get(entry); > > spin_unlock(&tree->lock); > > > > - ret = zswap_writeback_entry(entry, tree); > > + writeback_result = zswap_writeback_entry(entry, tree); > > > > spin_lock(&tree->lock); > > - if (ret) { > > - /* Writeback failed, put entry back on LRU */ > > - spin_lock(&pool->lru_lock); > > - list_move(&entry->lru, &pool->lru); > > - spin_unlock(&pool->lru_lock); > > + if (writeback_result) { > > + zswap_reject_reclaim_fail++; > > + memcg = get_mem_cgroup_from_entry(entry); > > Can this return NULL? Seems like we don't check the return in most/all places. I believe so, but memcg experts should fact check me on this. It's roughly the same pattern as zswap charging/uncharging: obj_cgroup_uncharge_zswap(entry->objcg, entry->length) -> getting memcg (under rcu_read_lock()) > > > + spin_lock(lock); > > + /* we cannot use zswap_lru_add here, because it increments node's lru count */ > > + list_lru_putback(&entry->pool->list_lru, item, entry_to_nid(entry), memcg); > > Perhaps we can move this call with the memcg get/put to a helper like > add/del? (e.g. zswap_lru_putback) > > We would need to move get_mem_cgroup_from_entry() into the lock but I > think that's okay. We probably could, but that sounds like extra code for not a lot of gains, no? > > > + spin_unlock(lock); > > + mem_cgroup_put(memcg); > > + ret = LRU_RETRY; > > goto put_unlock; > > } > > + zswap_written_back_pages++; > > > > /* > > * Writeback started successfully, the page now belongs to the > > @@ -687,7 +723,34 @@ static int zswap_reclaim_entry(struct zswap_pool *pool) > > zswap_entry_put(tree, entry); > > unlock: > > spin_unlock(&tree->lock); > > - return ret ? -EAGAIN : 0; > > + spin_lock(lock); > > + return ret; > > +} > > + > > +static int shrink_memcg(struct mem_cgroup *memcg) > > +{ > > + struct zswap_pool *pool; > > + int nid, shrunk = 0; > > + > > + /* > > + * Skip zombies because their LRUs are reparented and we would be > > + * reclaiming from the parent instead of the dead memcg. > > + */ > > + if (memcg && !mem_cgroup_online(memcg)) > > + return -ENOENT; > > + > > + pool = zswap_pool_current_get(); > > + if (!pool) > > + return -EINVAL; > > + > > + for_each_node_state(nid, N_NORMAL_MEMORY) { > > + unsigned long nr_to_walk = 1; > > + > > + shrunk += list_lru_walk_one(&pool->list_lru, nid, memcg, > > + &shrink_memcg_cb, NULL, &nr_to_walk); > > + } > > + zswap_pool_put(pool); > > + return shrunk ? 0 : -EAGAIN; > > } > > > > static void shrink_worker(struct work_struct *w) > > @@ -696,15 +759,17 @@ static void shrink_worker(struct work_struct *w) > > shrink_work); > > int ret, failures = 0; > > > > + /* global reclaim will select cgroup in a round-robin fashion. */ > > do { > > - ret = zswap_reclaim_entry(pool); > > - if (ret) { > > - zswap_reject_reclaim_fail++; > > - if (ret != -EAGAIN) > > - break; > > - if (++failures == MAX_RECLAIM_RETRIES) > > - break; > > - } > > + pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL); > > I think this can be a problem. We hold a ref to a memcg here until the > next time we shrink, which can be a long time IIUC. This can cause the > memcg to linger as a zombie. I understand it is one memcg per-zswap > pool, but I am still unsure about it. > > MGLRU maintains a memcg LRU for global reclaim that gets properly > cleaned up when a memcg is going away, so that's one option, although > complicated. > > A second option would be to hold a pointer to the objcg instead, which > should be less problematic (although we are still holding that objcg > hostage indefinitely). The problem here is that if the objcg gets > reparented, next time we will start at the parent of the memcg we > stopped at last time, which tbh doesn't sound bad at all to me. > > A third option would be to flag the memcg such that when it is getting > offlined we can call into zswap to reset pool->next_shrink (or move it > to the parent) and drop the ref. Although synchronization can get > hairy when racing with offlining. > > Not sure what's the right solution, but I prefer we don't hold any > memcgs hostages indefinitely. I also think if we end up using > mem_cgroup_iter() then there should be a mem_cgroup_iter_break() > somewhere if/when breaking the iteration. > I'm not sure if this is that big of a problem in the first place, but if it is, doing something similar to MGLRU is probably the cleanest: when the memcg is freed, trigger the zswap_exit_memcg() callback, which will loop through all the zswap pools and update pool->next_shrink where appropriate. Note that we only have one pool per (compression algorithm x allocator) combinations, so there cannot be that many pools, correct? Johannes suggests this idea to me (my apologies if I butcher it) during one of our conversations. That sounds relatively easy IIUC. > > + > > + ret = shrink_memcg(pool->next_shrink); > > + > > + if (ret == -EINVAL) > > + break; > > + if (ret && ++failures == MAX_RECLAIM_RETRIES) > > + break; > > + > > cond_resched(); > > } while (!zswap_can_accept()); > > zswap_pool_put(pool); > [..] > > @@ -1233,15 +1301,15 @@ bool zswap_store(struct folio *folio) > > zswap_invalidate_entry(tree, dupentry); > > } > > spin_unlock(&tree->lock); > > - > > - /* > > - * XXX: zswap reclaim does not work with cgroups yet. Without a > > - * cgroup-aware entry LRU, we will push out entries system-wide based on > > - * local cgroup limits. > > - */ > > objcg = get_obj_cgroup_from_folio(folio); > > - if (objcg && !obj_cgroup_may_zswap(objcg)) > > - goto reject; > > + if (objcg && !obj_cgroup_may_zswap(objcg)) { > > + memcg = get_mem_cgroup_from_objcg(objcg); > > + if (shrink_memcg(memcg)) { > > + mem_cgroup_put(memcg); > > + goto reject; > > + } > > + mem_cgroup_put(memcg); > > Here we choose to replicate mem_cgroup_put(). > > > + } > > > > /* reclaim space if needed */ > > if (zswap_is_full()) { > > @@ -1258,7 +1326,7 @@ bool zswap_store(struct folio *folio) > > } > > > > /* allocate entry */ > > - entry = zswap_entry_cache_alloc(GFP_KERNEL); > > + entry = zswap_entry_cache_alloc(GFP_KERNEL, page_to_nid(page)); > > if (!entry) { > > zswap_reject_kmemcache_fail++; > > goto reject; > > @@ -1285,6 +1353,15 @@ bool zswap_store(struct folio *folio) > > if (!entry->pool) > > goto freepage; > > > > + if (objcg) { > > + memcg = get_mem_cgroup_from_objcg(objcg); > > + lru_alloc_ret = memcg_list_lru_alloc(memcg, &entry->pool->list_lru, GFP_KERNEL); > > + mem_cgroup_put(memcg); > > + > > + if (lru_alloc_ret) > > + goto put_pool; > > + } > > Yet here we choose to have a single mem_cgroup_put() and stash the > output in a variable. > > Consistency would be nice. No strong opinions here, but yeah we should fix it to make it consistent. > > > + > > /* compress */ > > acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx); > > > > @@ -1361,9 +1438,8 @@ bool zswap_store(struct folio *folio) > > zswap_invalidate_entry(tree, dupentry); > > } > > if (entry->length) { > > - spin_lock(&entry->pool->lru_lock); > > - list_add(&entry->lru, &entry->pool->lru); > > - spin_unlock(&entry->pool->lru_lock); > > + INIT_LIST_HEAD(&entry->lru); > > + zswap_lru_add(&entry->pool->list_lru, entry); > > } > > spin_unlock(&tree->lock); > > > > @@ -1376,6 +1452,7 @@ bool zswap_store(struct folio *folio) > > > > put_dstmem: > > mutex_unlock(acomp_ctx->mutex); > > +put_pool: > > zswap_pool_put(entry->pool); > > freepage: > > zswap_entry_cache_free(entry); > > @@ -1470,9 +1547,8 @@ bool zswap_load(struct folio *folio) > > zswap_invalidate_entry(tree, entry); > > folio_mark_dirty(folio); > > } else if (entry->length) { > > - spin_lock(&entry->pool->lru_lock); > > - list_move(&entry->lru, &entry->pool->lru); > > - spin_unlock(&entry->pool->lru_lock); > > + zswap_lru_del(&entry->pool->list_lru, entry); > > + zswap_lru_add(&entry->pool->list_lru, entry); > > Can we use list_move_tail() here? (perhaps wrapped in a helper if needed). Maybe zswap_lru_move_tail()? :) FWIW, list_lru() interface does not have a move_tail function, (weird, I know). So this seems to be the common pattern for LRU rotation with list_lru. > > > } > > zswap_entry_put(tree, entry); > > spin_unlock(&tree->lock); > > -- > > 2.34.1
