Re: [PATCH] zsmalloc: move LRU update from zs_map_object() to zs_malloc()

[Nhat Pham]

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.