Message-ID: <CAGsJ_4wss6++EP8hSyuc3sPqtLgqqjs7DOUfQ5aCK36Bn+VbBg@mail.gmail.com>
Date: Fri, 14 Jun 2024 20:56:00 +1200
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, chrisl@...nel.org, 
	linux-kernel@...r.kernel.org, mhocko@...e.com, ryan.roberts@....com, 
	baolin.wang@...ux.alibaba.com, yosryahmed@...gle.com, shy828301@...il.com, 
	surenb@...gle.com, v-songbaohua@...o.com, willy@...radead.org, 
	ying.huang@...el.com, yuzhao@...gle.com
Subject: Re: [PATCH RFC 3/3] mm: remove folio_test_anon(folio)==false path in __folio_add_anon_rmap()

On Fri, Jun 14, 2024 at 8:51 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 14.06.24 09:56, Barry Song wrote:
> > On Thu, Jun 13, 2024 at 9:12 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 13.06.24 11:06, Barry Song wrote:
> >>> On Thu, Jun 13, 2024 at 8:49 PM David Hildenbrand <david@...hat.com> wrote:
> >>>>
> >>>> On 13.06.24 10:46, Barry Song wrote:
> >>>>> On Thu, Jun 13, 2024 at 12:08 PM Barry Song <21cnbao@...il.com> wrote:
> >>>>>>
> >>>>>> From: Barry Song <v-songbaohua@...o.com>
> >>>>>>
> >>>>>> The folio_test_anon(folio)==false case within do_swap_page() has been
> >>>>>> relocated to folio_add_new_anon_rmap(). Additionally, two other callers
> >>>>>> consistently pass anonymous folios.
> >>>>>>
> >>>>>> stack 1:
> >>>>>> remove_migration_pmd
> >>>>>>       -> folio_add_anon_rmap_pmd
> >>>>>>         -> __folio_add_anon_rmap
> >>>>>>
> >>>>>> stack 2:
> >>>>>> __split_huge_pmd_locked
> >>>>>>       -> folio_add_anon_rmap_ptes
> >>>>>>          -> __folio_add_anon_rmap
> >>>>>>
> >>>>>> __folio_add_anon_rmap() only needs to handle the cases
> >>>>>> folio_test_anon(folio)==true now.
> >>>>>
> >>>>> My team reported a case where swapoff() is calling
> >>>>> folio_add_anon_rmap_pte *not* folio_add_anon_rmap_ptes
> >>>>> with one new anon  (!folio_test_anon(folio)).
> >>>>>
> >>>>> I will double check all folio_add_anon_rmap_pte() cases.
> >>>>
> >>>> Right, swapoff() path is a bit special (e.g., we don't do any kind of
> >>>> batching on the swapoff() path).
> >>>>
> >>>> But we should never get a new large anon folio there, or could that now
> >>>> happen?
> >>>
> >>> My team encountered the following issue while testing this RFC
> >>> series on real hardware. Let me take a look to identify the
> >>> problem.
> >>>
> >>> [  261.214195][T11285] page:000000004cdd779e refcount:4 mapcount:1
> >>> mapping:00000000ba142c22 index:0x1 pfn:0x1b30a6
> >>> [  261.214213][T11285] memcg:ffffff8003678000
> >>> [  261.214217][T11285] aops:swap_aops
> >>> [  261.214233][T11285] flags:
> >>> 0x2000000000081009(locked|uptodate|owner_priv_1|swapbacked|zone=1|kasantag=0x0)
> >>> [  261.214241][T11285] page_type: 0x0()
> >>> [  261.214246][T11285] raw: 2000000000081009 0000000000000000
> >>> dead000000000122 0000000000000000
> >>> [  261.214251][T11285] raw: 0000000000000001 00000000000d84b3
> >>> 0000000400000000 ffffff8003678000
> >>> [  261.214254][T11285] page dumped because:
> >>> VM_WARN_ON_FOLIO(!folio_test_anon(folio))
> >>> [  261.214257][T11285] page_owner tracks the page as allocated
> >>> [  261.214260][T11285] page last allocated via order 0, migratetype
> >>> Movable, gfp_mask 0x2140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid
> >>> 11285, tgid 11285 (swapoff), ts 261214177545, free_ts 261151875699
> >>> [  261.214268][T11285]  post_alloc_hook+0x1b8/0x1c0
> >>> [  261.214284][T11285]  prep_new_page+0x28/0x13c
> >>> [  261.214291][T11285]  get_page_from_freelist+0x198c/0x1aa4
> >>> [  261.214298][T11285]  __alloc_pages+0x15c/0x330
> >>> [  261.214304][T11285]  __folio_alloc+0x1c/0x4c
> >>> [  261.214310][T11285]  __read_swap_cache_async+0xd8/0x48c
> >>> [  261.214320][T11285]  swap_cluster_readahead+0x158/0x324
> >>> [  261.214326][T11285]  swapin_readahead+0x64/0x448
> >>> [  261.214331][T11285]  __arm64_sys_swapoff+0x6ec/0x14b0
> >>> [  261.214337][T11285]  invoke_syscall+0x58/0x114
> >>> [  261.214353][T11285]  el0_svc_common+0xac/0xe0
> >>> [  261.214360][T11285]  do_el0_svc+0x1c/0x28
> >>> [  261.214366][T11285]  el0_svc+0x38/0x68
> >>> [  261.214372][T11285]  el0t_64_sync_handler+0x68/0xbc
> >>> [  261.214376][T11285]  el0t_64_sync+0x1a8/0x1ac
> >>> [  261.214381][T11285] page last free pid 90 tgid 90 stack trace:
> >>> [  261.214386][T11285]  free_unref_page_prepare+0x338/0x374
> >>> [  261.214395][T11285]  free_unref_page_list+0x84/0x378
> >>> [  261.214400][T11285]  shrink_folio_list+0x1234/0x13e4
> >>> [  261.214409][T11285]  evict_folios+0x1458/0x19b4
> >>> [  261.214417][T11285]  try_to_shrink_lruvec+0x1c8/0x264
> >>> [  261.214422][T11285]  shrink_one+0xa8/0x234
> >>> [  261.214427][T11285]  shrink_node+0xb38/0xde0
> >>> [  261.214432][T11285]  balance_pgdat+0x7a4/0xdb4
> >>> [  261.214437][T11285]  kswapd+0x290/0x4e4
> >>> [  261.214442][T11285]  kthread+0x114/0x1bc
> >>> [  261.214459][T11285]  ret_from_fork+0x10/0x20
> >>> [  261.214477][T11285] ------------[ cut here ]------------
> >>> [  261.214480][T11285] WARNING: CPU: 3 PID: 11285 at mm/rmap.c:1305
> >>> folio_add_anon_rmap_ptes+0x2b4/0x330
> >>>
> >>> [  261.215403][T11285] pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT
> >>> -SSBS BTYPE=--)
> >>> [  261.215423][T11285] pc : folio_add_anon_rmap_ptes+0x2b4/0x330
> >>> [  261.215428][T11285] lr : folio_add_anon_rmap_ptes+0x2b4/0x330
> >>> [  261.215433][T11285] sp : ffffffc0a7dbbbf0
> >>> [  261.215437][T11285] x29: ffffffc0a7dbbbf0 x28: ffffff8040860a08
> >>> x27: ffffff80db480000
> >>> [  261.215445][T11285] x26: fffffffe04cc2980 x25: ffffff808f77f120
> >>> x24: 0000007b44941000
> >>> [  261.215452][T11285] x23: 0000000000000001 x22: 0000000000000001
> >>> x21: fffffffe04cc2980
> >>> [  261.215459][T11285] x20: ffffff80db480000 x19: fffffffe04cc2980
> >>> x18: ffffffed011dae80
> >>> [  261.215465][T11285] x17: 0000000000000001 x16: ffffffffffffffff
> >>> x15: 0000000000000004
> >>> [  261.215471][T11285] x14: ffffff82fb73fac0 x13: 0000000000000003
> >>> x12: 0000000000000003
> >>> [  261.215476][T11285] x11: 00000000fffeffff x10: c0000000fffeffff x9
> >>> : 0d46c0889b468e00
> >>> [  261.215483][T11285] x8 : 0d46c0889b468e00 x7 : 545b5d3935343431 x6
> >>> : 322e31363220205b
> >>> [  261.215489][T11285] x5 : ffffffed013de407 x4 : ffffffed00698967 x3
> >>> : 0000000000000000
> >>> [  261.215495][T11285] x2 : 0000000000000000 x1 : ffffffc0a7dbb8c0 x0
> >>> : ffffff8068c15c80
> >>> [  261.215501][T11285] Call trace:
> >>> [  261.215504][T11285]  folio_add_anon_rmap_ptes+0x2b4/0x330
> >>> [  261.215509][T11285]  __arm64_sys_swapoff+0x8cc/0x14b0
> >>> [  261.215516][T11285]  invoke_syscall+0x58/0x114
> >>> [  261.215526][T11285]  el0_svc_common+0xac/0xe0
> >>> [  261.215532][T11285]  do_el0_svc+0x1c/0x28
> >>> [  261.215539][T11285]  el0_svc+0x38/0x68
> >>> [  261.215544][T11285]  el0t_64_sync_handler+0x68/0xbc
> >>> [  261.215548][T11285]  el0t_64_sync+0x1a8/0x1ac
> >>> [  261.215552][T11285] ---[ end trace 0000000000000000 ]---
> >>
> >> Ah, yes. in unuse_pte(), you'll have to do the right thing if
> >> !folio_test(anon) before doing the folio_add_anon_rmap_pte().
> >>
> >> You might have a fresh anon folio in the swapcache that was never mapped
> >> (hopefully order-0, otherwise we'd likely be in trouble).
> >
> > Yes. It is order-0
> >
> > [  261.214260][T11285] page last allocated via order 0, migratetype
> >
> > Otherwise, we would have encountered this VM_WARN_ON_FOLIO?
> >
> > __folio_add_anon_rmap()
> > {
> > ...
> > VM_WARN_ON_FOLIO(folio_test_large(folio) &&
> >       level != RMAP_LEVEL_PMD, folio);
> > ...
> > }
> >
> > Given that nobody has ever reported this warning, I assume all callers
> > using folio_add_anon_rmap_pte(s) are right now safe to move to ?
>
> Yes, and we should likely add a VM_WARN_ON_ONCE() here that we have a
> small folio if !anon. If that ever changes, we can assess the situation.
>

this patch actually has a WARN_ON for all !anon, so it extends the WARN
to small.

-       if (unlikely(!folio_test_anon(folio))) {
-               VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
-               /*
-                * For a PTE-mapped large folio, we only know that the single
-                * PTE is exclusive. Further, __folio_set_anon() might not get
-                * folio->index right when not given the address of the head
-                * page.
-                */
-               VM_WARN_ON_FOLIO(folio_test_large(folio) &&
-                                level != RMAP_LEVEL_PMD, folio);
-               __folio_set_anon(folio, vma, address,
-                                !!(flags & RMAP_EXCLUSIVE));
-       } else if (likely(!folio_test_ksm(folio))) {
+       VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
+

> Only swap created "new" anon folios without properly calling the right
> function so far, all other code handles that correctly.
>
> --
> Cheers,
>
> David / dhildenb
>

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