On Mon, May 8, 2023 at 7:07 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Sat, May 06, 2023 at 12:01:40PM +0900, Sergey Senozhatsky wrote: > > On (23/05/05 11:50), Nhat Pham wrote: > > > Under memory pressure, we sometimes observe the following crash: > > > > > > [ 5694.832838] ------------[ cut here ]------------ > > > [ 5694.842093] list_del corruption, ffff888014b6a448->next is LIST_POISON1 (dead000000000100) > > > [ 5694.858677] WARNING: CPU: 33 PID: 418824 at lib/list_debug.c:47 __list_del_entry_valid+0x42/0x80 > > > [ 5694.961820] CPU: 33 PID: 418824 Comm: fuse_counters.s Kdump: loaded Tainted: G S 5.19.0-0_fbk3_rc3_hoangnhatpzsdynshrv41_10870_g85a9558a25de #1 > > > [ 5694.990194] Hardware name: Wiwynn Twin Lakes MP/Twin Lakes Passive MP, BIOS YMM16 05/24/2021 > > > [ 5695.007072] RIP: 0010:__list_del_entry_valid+0x42/0x80 > > > [ 5695.017351] Code: 08 48 83 c2 22 48 39 d0 74 24 48 8b 10 48 39 f2 75 2c 48 8b 51 08 b0 01 48 39 f2 75 34 c3 48 c7 c7 55 d7 78 82 e8 4e 45 3b 00 <0f> 0b eb 31 48 c7 c7 27 a8 70 82 e8 3e 45 3b 00 0f 0b eb 21 48 c7 > > > [ 5695.054919] RSP: 0018:ffffc90027aef4f0 EFLAGS: 00010246 > > > [ 5695.065366] RAX: 41fe484987275300 RBX: ffff888008988180 RCX: 0000000000000000 > > > [ 5695.079636] RDX: ffff88886006c280 RSI: ffff888860060480 RDI: ffff888860060480 > > > [ 5695.093904] RBP: 0000000000000002 R08: 0000000000000000 R09: ffffc90027aef370 > > > [ 5695.108175] R10: 0000000000000000 R11: ffffffff82fdf1c0 R12: 0000000010000002 > > > [ 5695.122447] R13: ffff888014b6a448 R14: ffff888014b6a420 R15: 00000000138dc240 > > > [ 5695.136717] FS: 00007f23a7d3f740(0000) GS:ffff888860040000(0000) knlGS:0000000000000000 > > > [ 5695.152899] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > [ 5695.164388] CR2: 0000560ceaab6ac0 CR3: 000000001c06c001 CR4: 00000000007706e0 > > > [ 5695.178659] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > > > [ 5695.192927] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > > > [ 5695.207197] PKRU: 55555554 > > > [ 5695.212602] Call Trace: > > > [ 5695.217486] <TASK> > > > [ 5695.221674] zs_map_object+0x91/0x270 > > > [ 5695.229000] zswap_frontswap_store+0x33d/0x870 > > > [ 5695.237885] ? do_raw_spin_lock+0x5d/0xa0 > > > [ 5695.245899] __frontswap_store+0x51/0xb0 > > > [ 5695.253742] swap_writepage+0x3c/0x60 > > > [ 5695.261063] shrink_page_list+0x738/0x1230 > > > [ 5695.269255] shrink_lruvec+0x5ec/0xcd0 > > > [ 5695.276749] ? shrink_slab+0x187/0x5f0 > > > [ 5695.284240] ? mem_cgroup_iter+0x6e/0x120 > > > [ 5695.292255] shrink_node+0x293/0x7b0 > > > [ 5695.299402] do_try_to_free_pages+0xea/0x550 > > > [ 5695.307940] try_to_free_pages+0x19a/0x490 > > > [ 5695.316126] __folio_alloc+0x19ff/0x3e40 > > > [ 5695.323971] ? __filemap_get_folio+0x8a/0x4e0 > > > [ 5695.332681] ? walk_component+0x2a8/0xb50 > > > [ 5695.340697] ? generic_permission+0xda/0x2a0 > > > [ 5695.349231] ? __filemap_get_folio+0x8a/0x4e0 > > > [ 5695.357940] ? walk_component+0x2a8/0xb50 > > > [ 5695.365955] vma_alloc_folio+0x10e/0x570 > > > [ 5695.373796] ? walk_component+0x52/0xb50 > > > [ 5695.381634] wp_page_copy+0x38c/0xc10 > > > [ 5695.388953] ? filename_lookup+0x378/0xbc0 > > > [ 5695.397140] handle_mm_fault+0x87f/0x1800 > > > [ 5695.405157] do_user_addr_fault+0x1bd/0x570 > > > [ 5695.413520] exc_page_fault+0x5d/0x110 > > > [ 5695.421017] asm_exc_page_fault+0x22/0x30 > > > > > > After some investigation, I have found the following issue: unlike other > > > zswap backends, zsmalloc performs the LRU list update at the object > > > mapping time, rather than when the slot for the object is allocated. > > > This deviation was discussed and agreed upon during the review process > > > of the zsmalloc writeback patch series: > > > > > > https://lore.kernel.org/lkml/Y3flcAXNxxrvy3ZH@xxxxxxxxxxx/ > > > > > > Unfortunately, this introduces a subtle bug that occurs when there is a > > > concurrent store and reclaim, which interleave as follows: > > > > > > zswap_frontswap_store() shrink_worker() > > > zs_malloc() zs_zpool_shrink() > > > spin_lock(&pool->lock) zs_reclaim_page() > > > zspage = find_get_zspage() > > > spin_unlock(&pool->lock) > > > spin_lock(&pool->lock) > > > zspage = list_first_entry(&pool->lru) > > > list_del(&zspage->lru) > > > zspage->lru.next = LIST_POISON1 > > > zspage->lru.prev = LIST_POISON2 > > > > Will list_del_init() there do the trick? > > > > > spin_unlock(&pool->lock) > > > zs_map_object() > > > spin_lock(&pool->lock) > > > if (!list_empty(&zspage->lru)) > > > list_del(&zspage->lru) > > > > list_del_init() > > The deeper bug here is that zs_map_object() tries to add the page to > the LRU list while the shrinker has it isolated for reclaim. This is > way too sutble and error prone. Even if it worked now, it'll cause > corruption issues down the line. > > For example, Nhat is adding a secondary entry point to reclaim. > Reclaim expects that a page that's on the LRU is also on the fullness > list, so this would lead to a double remove_zspage() and BUG_ON(). > > This patch doesn't just fix the crash, it eliminates the deeper LRU > isolation issue and makes the code more robust and simple. I agree. IMO, less unnecessary concurrent interaction is always a win for developers' and maintainers' cognitive load. As a side benefit - this also gets rid of the inelegant check (mm == ZS_MM_WO). The fact that we had to include a a multi-paragraph explanation for a 3-line piece of code should have been a red flag.
