Re: [PATCH v3 13/20] mm: Copy-on-Write (COW) reuse support for PTE-mapped THP

Kairui Song
2025-04-19T16:33:53Z

On Sun, Apr 20, 2025 at 12:26 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 19.04.25 18:02, Kairui Song wrote:
> > On Tue, Mar 4, 2025 at 12:46 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> Currently, we never end up reusing PTE-mapped THPs after fork. This
> >> wasn't really a problem with PMD-sized THPs, because they would have to
> >> be PTE-mapped first, but it's getting a problem with smaller THP
> >> sizes that are effectively always PTE-mapped.
> >>
> >> With our new "mapped exclusively" vs "maybe mapped shared" logic for
> >> large folios, implementing CoW reuse for PTE-mapped THPs is straight
> >> forward: if exclusively mapped, make sure that all references are
> >> from these (our) mappings. Add some helpful comments to explain the
> >> details.
> >>
> >> CONFIG_TRANSPARENT_HUGEPAGE selects CONFIG_MM_ID. If we spot an anon
> >> large folio without CONFIG_TRANSPARENT_HUGEPAGE in that code, something
> >> is seriously messed up.
> >>
> >> There are plenty of things we can optimize in the future: For example, we
> >> could remember that the folio is fully exclusive so we could speedup
> >> the next fault further. Also, we could try "faulting around", turning
> >> surrounding PTEs that map the same folio writable. But especially the
> >> latter might increase COW latency, so it would need further
> >> investigation.
> >>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >> ---
> >> mm/memory.c | 83 +++++++++++++++++++++++++++++++++++++++++++++++------
> >> 1 file changed, 75 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index 73b783c7d7d51..bb245a8fe04bc 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -3729,19 +3729,86 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf, struct folio *folio)
> >> return ret;
> >> }
> >>
> >> -static bool wp_can_reuse_anon_folio(struct folio *folio,
> >> - struct vm_area_struct *vma)
> >> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >> +static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
> >> + struct vm_area_struct *vma)
> >> {
> >> + bool exclusive = false;
> >> +
> >> + /* Let's just free up a large folio if only a single page is mapped. */
> >> + if (folio_large_mapcount(folio) <= 1)
> >> + return false;
> >> +
> >> /*
> >> - * We could currently only reuse a subpage of a large folio if no
> >> - * other subpages of the large folios are still mapped. However,
> >> - * let's just consistently not reuse subpages even if we could
> >> - * reuse in that scenario, and give back a large folio a bit
> >> - * sooner.
> >> + * The assumption for anonymous folios is that each page can only get
> >> + * mapped once into each MM. The only exception are KSM folios, which
> >> + * are always small.
> >> + *
> >> + * Each taken mapcount must be paired with exactly one taken reference,
> >> + * whereby the refcount must be incremented before the mapcount when
> >> + * mapping a page, and the refcount must be decremented after the
> >> + * mapcount when unmapping a page.
> >> + *
> >> + * If all folio references are from mappings, and all mappings are in
> >> + * the page tables of this MM, then this folio is exclusive to this MM.
> >> */
> >> - if (folio_test_large(folio))
> >> + if (folio_test_large_maybe_mapped_shared(folio))
> >> + return false;
> >> +
> >> + VM_WARN_ON_ONCE(folio_test_ksm(folio));
> >> + VM_WARN_ON_ONCE(folio_mapcount(folio) > folio_nr_pages(folio));
> >> + VM_WARN_ON_ONCE(folio_entire_mapcount(folio));
> >> +
> >> + if (unlikely(folio_test_swapcache(folio))) {
> >> + /*
> >> + * Note: freeing up the swapcache will fail if some PTEs are
> >> + * still swap entries.
> >> + */
> >> + if (!folio_trylock(folio))
> >> + return false;
> >> + folio_free_swap(folio);
> >> + folio_unlock(folio);
> >> + }
> >> +
> >> + if (folio_large_mapcount(folio) != folio_ref_count(folio))
> >> return false;
> >>
> >> + /* Stabilize the mapcount vs. refcount and recheck. */
> >> + folio_lock_large_mapcount(folio);
> >> + VM_WARN_ON_ONCE(folio_large_mapcount(folio) < folio_ref_count(folio));
> >
> > Hi David, I'm seeing this WARN_ON being triggered on my test machine:
>
> Hi!
>
> So I assume the following will not sort out the issue for you, correct?
>
> https://lore.kernel.org/all/20250415095007.569836-1-david@redhat.com/T/#u

Yes, double checked the commit I'm testing
dc683247117ee018e5da6b04f1c499acdc2a1418 (akpm/mm-unstable) includes
this fix.

>
> >
> > I'm currently working on my swap table series and testing heavily with
> > swap related workloads. I thought my patch may break the kernel, but
> > after more investigation and reverting to current mm-unstable, it
> > still occurs (with a much lower chance though, I think my series
> > changed the timing so it's more frequent in my case).
> >
> > The test is simple, I just enable all mTHP sizes and repeatedly build
> > linux kernel in a 1G memcg using tmpfs.
> >
> > The WARN is reproducible with current mm-unstable
> > (dc683247117ee018e5da6b04f1c499acdc2a1418):
> >
> > [ 5268.100379] ------------[ cut here ]------------
> > [ 5268.105925] WARNING: CPU: 2 PID: 700274 at mm/memory.c:3792
> > do_wp_page+0xfc5/0x1080
> > [ 5268.112437] Modules linked in: zram virtiofs
> > [ 5268.115507] CPU: 2 UID: 0 PID: 700274 Comm: cc1 Kdump: loaded Not
> > tainted 6.15.0-rc2.ptch-gdc683247117e #1434 PREEMPT(voluntary)
> > [ 5268.120562] Hardware name: Red Hat KVM/RHEL-AV, BIOS 0.0.0 02/06/2015
> > [ 5268.123025] RIP: 0010:do_wp_page+0xfc5/0x1080
> > [ 5268.124807] Code: 0d 80 77 32 02 0f 85 3e f1 ff ff 0f 1f 44 00 00
> > e9 34 f1 ff ff 48 0f ba 75 00 1f 65 ff 0d 63 77 32 02 0f 85 21 f1 ff
> > ff eb e1 <0f> 0b e9 10 fd ff ff 65 ff 00 f0 48 0f b
> > a 6d 00 1f 0f 83 ec fc ff
> > [ 5268.132034] RSP: 0000:ffffc900234efd48 EFLAGS: 00010297
> > [ 5268.134002] RAX: 0000000000000080 RBX: 0000000000000000 RCX: 000fffffffe00000
> > [ 5268.136609] RDX: 0000000000000081 RSI: 00007f009cbad000 RDI: ffffea0012da0000
> > [ 5268.139371] RBP: ffffea0012da0068 R08: 80000004b682d025 R09: 00007f009c7c0000
> > [ 5268.142183] R10: ffff88839c48b8c0 R11: 0000000000000000 R12: ffff88839c48b8c0
> > [ 5268.144738] R13: ffffea0012da0000 R14: 00007f009cbadf10 R15: ffffc900234efdd8
> > [ 5268.147540] FS: 00007f009d1fdac0(0000) GS:ffff88a07ae14000(0000)
> > knlGS:0000000000000000
> > [ 5268.150715] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 5268.153270] CR2: 00007f009cbadf10 CR3: 000000016c7c0001 CR4: 0000000000770eb0
> > [ 5268.155674] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > [ 5268.158100] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > [ 5268.160613] PKRU: 55555554
> > [ 5268.161662] Call Trace:
> > [ 5268.162609] <TASK>
> > [ 5268.163438] ? ___pte_offset_map+0x1b/0x110
> > [ 5268.165309] __handle_mm_fault+0xa51/0xf00
> > [ 5268.166848] ? update_load_avg+0x80/0x760
> > [ 5268.168376] handle_mm_fault+0x13d/0x360
> > [ 5268.169930] do_user_addr_fault+0x2f2/0x7f0
> > [ 5268.171630] exc_page_fault+0x6a/0x140
> > [ 5268.173278] asm_exc_page_fault+0x26/0x30
> > [ 5268.174866] RIP: 0033:0x120e8e4
> > [ 5268.176272] Code: 84 a9 00 00 00 48 39 c3 0f 85 ae 00 00 00 48 8b
> > 43 20 48 89 45 38 48 85 c0 0f 85 b7 00 00 00 48 8b 43 18 48 8b 15 6c
> > 08 42 01 <0f> 11 43 10 48 89 1d 61 08 42 01 48 89 53 18 0f 11 03 0f 11
> > 43 20
> > [ 5268.184121] RSP: 002b:00007fff8a855160 EFLAGS: 00010246
> > [ 5268.186343] RAX: 00007f009cbadbd0 RBX: 00007f009cbadf00 RCX: 0000000000000000
> > [ 5268.189209] RDX: 00007f009cbba030 RSI: 00000000000006f4 RDI: 0000000000000000
> > [ 5268.192145] RBP: 00007f009cbb6460 R08: 00007f009d10f000 R09: 000000000000016c
> > [ 5268.194687] R10: 0000000000000000 R11: 0000000000000010 R12: 00007f009cf97660
> > [ 5268.197172] R13: 00007f009756ede0 R14: 00007f0097582348 R15: 0000000000000002
> > [ 5268.199419] </TASK>
> > [ 5268.200227] ---[ end trace 0000000000000000 ]---
> >
> > I also once changed the WARN_ON to WARN_ON_FOLIO and I got more info here:
> >
> > [ 3994.907255] page: refcount:9 mapcount:1 mapping:0000000000000000
> > index:0x7f90b3e98 pfn:0x615028
> > [ 3994.914449] head: order:3 mapcount:8 entire_mapcount:0
> > nr_pages_mapped:8 pincount:0
> > [ 3994.924534] memcg:ffff888106746000
> > [ 3994.927868] anon flags:
> > 0x17ffffc002084c(referenced|uptodate|owner_2|head|swapbacked|node=0|zone=2|lastcpupid=0x1fffff)
> > [ 3994.933479] raw: 0017ffffc002084c ffff88816edd9128 ffffea000beac108
> > ffff8882e8ba6bc9
> > [ 3994.936251] raw: 00000007f90b3e98 0000000000000000 0000000900000000
> > ffff888106746000
> > [ 3994.939466] head: 0017ffffc002084c ffff88816edd9128
> > ffffea000beac108 ffff8882e8ba6bc9
> > [ 3994.943355] head: 00000007f90b3e98 0000000000000000
> > 0000000900000000 ffff888106746000
> > [ 3994.946988] head: 0017ffffc0000203 ffffea0018540a01
> > 0000000800000007 00000000ffffffff
> > [ 3994.950328] head: ffffffff00000007 00000000800000a3
> > 0000000000000000 0000000000000008
> > [ 3994.953684] page dumped because:
> > VM_WARN_ON_FOLIO(folio_large_mapcount(folio) < folio_ref_count(folio))
> > [ 3994.957534] ------------[ cut here ]------------
> > [ 3994.959917] WARNING: CPU: 16 PID: 555282 at mm/memory.c:3794
> > do_wp_page+0x10c0/0x1110
> > [ 3994.963069] Modules linked in: zram virtiofs
> > [ 3994.964726] CPU: 16 UID: 0 PID: 555282 Comm: sh Kdump: loaded Not
> > tainted 6.15.0-rc1.ptch-ge39aef85f4c0-dirty #1431 PREEMPT(voluntary)
> > [ 3994.969985] Hardware name: Red Hat KVM/RHEL-AV, BIOS 0.0.0 02/06/2015
> > [ 3994.972905] RIP: 0010:do_wp_page+0x10c0/0x1110
> > [ 3994.974477] Code: fe ff 0f 0b bd f5 ff ff ff e9 16 fb ff ff 41 83
> > a9 bc 12 00 00 01 e9 2f fb ff ff 48 c7 c6 90 c2 49 82 4c 89 ef e8 40
> > fd fe ff <0f> 0b e9 6a fc ff ff 65 ff 00 f0 48 0f b
> > a 6d 00 1f 0f 83 46 fc ff
> > [ 3994.981033] RSP: 0000:ffffc9002b3c7d40 EFLAGS: 00010246
> > [ 3994.982636] RAX: 000000000000005b RBX: 0000000000000000 RCX: 0000000000000000
> > [ 3994.984778] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff889ffea16a80
> > [ 3994.986865] RBP: ffffea0018540a68 R08: 0000000000000000 R09: c0000000ffff7fff
> > [ 3994.989316] R10: 0000000000000001 R11: ffffc9002b3c7b80 R12: ffff88810cfd7d40
> > [ 3994.991654] R13: ffffea0018540a00 R14: 00007f90b3e9d620 R15: ffffc9002b3c7dd8
> > [ 3994.994076] FS: 00007f90b3caa740(0000) GS:ffff88a07b194000(0000)
> > knlGS:0000000000000000
> > [ 3994.996939] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 3994.998902] CR2: 00007f90b3e9d620 CR3: 0000000104088004 CR4: 0000000000770eb0
> > [ 3995.001314] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > [ 3995.003746] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > [ 3995.006173] PKRU: 55555554
> > [ 3995.007117] Call Trace:
> > [ 3995.007988] <TASK>
> > [ 3995.008755] ? __pfx_default_wake_function+0x10/0x10
> > [ 3995.010490] ? ___pte_offset_map+0x1b/0x110
> > [ 3995.011929] __handle_mm_fault+0xa51/0xf00
> > [ 3995.013346] handle_mm_fault+0x13d/0x360
> > [ 3995.014796] do_user_addr_fault+0x2f2/0x7f0
> > [ 3995.016331] ? sigprocmask+0x77/0xa0
> > [ 3995.017656] exc_page_fault+0x6a/0x140
> > [ 3995.018978] asm_exc_page_fault+0x26/0x30
> > [ 3995.020309] RIP: 0033:0x7f90b3d881a7
> > [ 3995.021461] Code: e8 4e b1 f8 ff 66 66 2e 0f 1f 84 00 00 00 00 00
> > 0f 1f 00 f3 0f 1e fa 55 31 c0 ba 01 00 00 00 48 89 e5 53 48 89 fb 48
> > 83 ec 08 <f0> 0f b1 15 71 54 11 00 0f 85 3b 01 00 0
> > 0 48 8b 35 84 54 11 00 48
> > [ 3995.028091] RSP: 002b:00007ffc33632c90 EFLAGS: 00010206
> > [ 3995.029992] RAX: 0000000000000000 RBX: 0000560cfbfc0a40 RCX: 0000000000000000
> > [ 3995.032456] RDX: 0000000000000001 RSI: 0000000000000005 RDI: 0000560cfbfc0a40
> > [ 3995.034794] RBP: 00007ffc33632ca0 R08: 00007ffc33632d50 R09: 00007ffc33632cff
> > [ 3995.037534] R10: 00007ffc33632c70 R11: 00007ffc33632d00 R12: 0000560cfbfc0a40
> > [ 3995.041063] R13: 00007f90b3e97fd0 R14: 00007f90b3e97fa8 R15: 0000000000000000
> > [ 3995.044390] </TASK>
> > [ 3995.045510] ---[ end trace 0000000000000000 ]---
> >
> > My guess is folio_ref_count is not a reliable thing to check here,
> > anything can increase the folio's ref account even without locking it,
> > for example, a swap cache lookup or maybe anything iterating the LRU.
>
> It is reliable, we are holding the mapcount lock, so for each mapcount
> we must have a corresponding refcount. If that is not the case, we have
> an issue elsewhere.

Yes, each mapcount should have a refcount, but could there be any
other thing to increase the refcount?

The check is: folio_large_mapcount(folio) < folio_ref_count(folio), so
maybe something just called folio_try_get and then put the folio
quickly.

>
> Other reference may only increase the refcount, but not violate the
> mapcount vs. refcount condition.
>
> Can you reproduce also with swap disabled?

I'm not sure how to reproduce it else way, I only saw this while the
swap stress test.

>
> --
> Cheers,
>
> David / dhildenb
>