Re: [PATCH v5 6/6] mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD

From: Ryan Roberts
Date: Tue Apr 02 2024 - 06:56:38 EST

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In reply to: Barry Song: "Re: [PATCH v5 6/6] mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD"
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On 02/04/2024 11:16, Barry Song wrote:
> On Thu, Mar 28, 2024 at 3:46 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> Rework madvise_cold_or_pageout_pte_range() to avoid splitting any large
>> folio that is fully and contiguously mapped in the pageout/cold vm
>> range. This change means that large folios will be maintained all the
>> way to swap storage. This both improves performance during swap-out, by
>> eliding the cost of splitting the folio, and sets us up nicely for
>> maintaining the large folio when it is swapped back in (to be covered in
>> a separate series).
>>
>> Folios that are not fully mapped in the target range are still split,
>> but note that behavior is changed so that if the split fails for any
>> reason (folio locked, shared, etc) we now leave it as is and move to the
>> next pte in the range and continue work on the proceeding folios.
>> Previously any failure of this sort would cause the entire operation to
>> give up and no folios mapped at higher addresses were paged out or made
>> cold. Given large folios are becoming more common, this old behavior
>> would have likely lead to wasted opportunities.
>>
>> While we are at it, change the code that clears young from the ptes to
>> use ptep_test_and_clear_young(), via the new mkold_ptes() batch helper
>> function. This is more efficent than get_and_clear/modify/set,
>> especially for contpte mappings on arm64, where the old approach would
>> require unfolding/refolding and the new approach can be done in place.
>>
>> Reviewed-by: Barry Song <v-songbaohua@xxxxxxxx>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>
> Hi Ryan,
>
> I'm not entirely certain if this issue is related to this patch, but
> I've encountered
> the KNIC twice while using the latest mm-unstable kernel. Each time I attempted
> to debug it, the issue vanished. I'm posting here to see if you have
> any ideas on it :-)

Thanks for the report! I think I see the problem below...

>
> [ 50.444066]
> [ 50.444495] =====================================
> [ 50.444954] WARNING: bad unlock balance detected!
> [ 50.445443] 6.9.0-rc2-00257-g2d9f63c285db #128 Not tainted
> [ 50.446233] -------------------------------------
> [ 50.446684] singlethread/102 is trying to release lock
> (ptlock_ptr(ptdesc)) at:
> [ 50.447635] [<ffffb03155fe302c>]
> madvise_cold_or_pageout_pte_range+0x80c/0xea0
> [ 50.449066] but there are no more locks to release!
> [ 50.449535]
> [ 50.449535] other info that might help us debug this:
> [ 50.450140] 1 lock held by singlethread/102:
> [ 50.450688] #0: ffff0000c001f208 (&mm->mmap_lock){++++}-{4:4}, at:
> do_madvise.part.0+0x178/0x518
> [ 50.452321]
> [ 50.452321] stack backtrace:
> [ 50.452959] CPU: 3 PID: 102 Comm: singlethread Not tainted
> 6.9.0-rc2-00257-g2d9f63c285db #128
> [ 50.453812] Hardware name: linux,dummy-virt (DT)
> [ 50.454373] Call trace:
> [ 50.454755] dump_backtrace+0x9c/0x100
> [ 50.455246] show_stack+0x20/0x38
> [ 50.455667] dump_stack_lvl+0xec/0x150
> [ 50.456111] dump_stack+0x18/0x28
> [ 50.456533] print_unlock_imbalance_bug+0x130/0x148
> [ 50.457014] lock_release+0x2e0/0x360
> [ 50.457487] _raw_spin_unlock+0x2c/0x78
> [ 50.457997] madvise_cold_or_pageout_pte_range+0x80c/0xea0
> [ 50.458635] walk_pgd_range+0x388/0x7d8
> [ 50.459168] __walk_page_range+0x1e0/0x1f0
> [ 50.459682] walk_page_range+0x1f0/0x2c8
> [ 50.460225] madvise_pageout+0xf8/0x280
> [ 50.460711] madvise_vma_behavior+0x310/0x9b8
> [ 50.461169] madvise_walk_vmas+0xc0/0x128
> [ 50.461605] do_madvise.part.0+0xf8/0x518
> [ 50.462041] __arm64_sys_madvise+0x68/0x88
> [ 50.462529] invoke_syscall+0x50/0x128
> [ 50.463001] el0_svc_common.constprop.0+0x48/0xf8
> [ 50.463508] do_el0_svc+0x28/0x40
> [ 50.464004] el0_svc+0x50/0x150
> [ 50.464492] el0t_64_sync_handler+0x13c/0x158
> [ 50.465021] el0t_64_sync+0x1a4/0x1a8
> [ 50.466959] ------------[ cut here ]------------
> [ 50.467451] WARNING: CPU: 3 PID: 102 at
> kernel/rcu/tree_plugin.h:431 __rcu_read_unlock+0x74/0x218
> [ 50.468160] Modules linked in:
> [ 50.468803] CPU: 3 PID: 102 Comm: singlethread Not tainted
> 6.9.0-rc2-00257-g2d9f63c285db #128
> [ 50.469658] Hardware name: linux,dummy-virt (DT)
> [ 50.470293] pstate: a3400005 (NzCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
> [ 50.470991] pc : __rcu_read_unlock+0x74/0x218
> [ 50.471594] lr : madvise_cold_or_pageout_pte_range+0x828/0xea0
> [ 50.472236] sp : ffff800080abb7e0
> [ 50.472622] pmr_save: 000000e0
> [ 50.473010] x29: ffff800080abb7e0 x28: 0000ffffa467a000 x27: fffffdffc3128c00
> [ 50.474006] x26: 0010000000000001 x25: 000000000000001b x24: ffff0000c32d73d0
> [ 50.474971] x23: 0060000104a3afc3 x22: ffff0000c2492840 x21: 0400000000000001
> [ 50.475943] x20: ff77fffffffffbff x19: ffff0000c3230000 x18: ffffffffffffffff
> [ 50.477286] x17: 672d37353230302d x16: 3263722d302e392e x15: ffff800100abb227
> [ 50.478373] x14: 0000000000000001 x13: 38613178302f3461 x12: 3178302b636e7973
> [ 50.479354] x11: fffffffffffe0000 x10: ffffb03159697d08 x9 : ffffb03155fe3048
> [ 50.480265] x8 : 00000000ffffefff x7 : ffffb03159697d08 x6 : 0000000000000000
> [ 50.481154] x5 : 0000000000000001 x4 : ffff800080abbfe0 x3 : 0000000000000000
> [ 50.482035] x2 : ffff4fd055074000 x1 : 00000000ffffffff x0 : 000000003fffffff
> [ 50.483163] Call trace:
> [ 50.483599] __rcu_read_unlock+0x74/0x218
> [ 50.484152] madvise_cold_or_pageout_pte_range+0x828/0xea0
> [ 50.484780] walk_pgd_range+0x388/0x7d8
> [ 50.485328] __walk_page_range+0x1e0/0x1f0
> [ 50.485725] walk_page_range+0x1f0/0x2c8
> [ 50.486117] madvise_pageout+0xf8/0x280
> [ 50.486547] madvise_vma_behavior+0x310/0x9b8
> [ 50.486975] madvise_walk_vmas+0xc0/0x128
> [ 50.487403] do_madvise.part.0+0xf8/0x518
> [ 50.487845] __arm64_sys_madvise+0x68/0x88
> [ 50.488374] invoke_syscall+0x50/0x128
> [ 50.488946] el0_svc_common.constprop.0+0x48/0xf8
> [ 50.489732] do_el0_svc+0x28/0x40
> [ 50.490210] el0_svc+0x50/0x150
> [ 50.490674] el0t_64_sync_handler+0x13c/0x158
> [ 50.491257] el0t_64_sync+0x1a4/0x1a8
> [ 50.491793] irq event stamp: 3087
> [ 50.492243] hardirqs last enabled at (3087): [<ffffb031570d89d8>]
> _raw_spin_unlock_irq+0x38/0x90
> [ 50.492917] hardirqs last disabled at (3086): [<ffffb031570d8acc>]
> _raw_spin_lock_irq+0x9c/0xc0
> [ 50.493742] softirqs last enabled at (2470): [<ffffb03155c10d94>]
> __do_softirq+0x534/0x588
> [ 50.494567] softirqs last disabled at (2461): [<ffffb03155c17238>]
> ____do_softirq+0x18/0x30
> [ 50.495328] ---[ end trace 0000000000000000 ]---
> [ 50.497110] BUG: sleeping function called from invalid context at
> kernel/locking/rwsem.c:1578
> [ 50.497544] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid:
> 102, name: singlethread
> [ 50.497652] preempt_count: ffffffff, expected: 0
> [ 50.497728] RCU nest depth: -1, expected: 0
> [ 50.497851] INFO: lockdep is turned off.
> [ 50.498023] CPU: 3 PID: 102 Comm: singlethread Tainted: G W
> 6.9.0-rc2-00257-g2d9f63c285db #128
> [ 50.498166] Hardware name: linux,dummy-virt (DT)
> [ 50.498221] Call trace:
> [ 50.498260] dump_backtrace+0x9c/0x100
> [ 50.498378] show_stack+0x20/0x38
> [ 50.498487] dump_stack_lvl+0xec/0x150
> [ 50.498574] dump_stack+0x18/0x28
> [ 50.498659] __might_resched+0x158/0x278
> [ 50.498741] __might_sleep+0x50/0xa0
> [ 50.498849] down_write+0x30/0x1a8
> [ 50.498950] split_huge_page_to_list_to_order+0x3c8/0x1130
> [ 50.499052] madvise_cold_or_pageout_pte_range+0x84c/0xea0
> [ 50.499138] walk_pgd_range+0x388/0x7d8
> [ 50.499224] __walk_page_range+0x1e0/0x1f0
> [ 50.499334] walk_page_range+0x1f0/0x2c8
> [ 50.499458] madvise_pageout+0xf8/0x280
> [ 50.499554] madvise_vma_behavior+0x310/0x9b8
> [ 50.499657] madvise_walk_vmas+0xc0/0x128
> [ 50.499739] do_madvise.part.0+0xf8/0x518
> [ 50.499851] __arm64_sys_madvise+0x68/0x88
> [ 50.499953] invoke_syscall+0x50/0x128
> [ 50.500037] el0_svc_common.constprop.0+0x48/0xf8
> [ 50.500121] do_el0_svc+0x28/0x40
> [ 50.500203] el0_svc+0x50/0x150
> [ 50.500322] el0t_64_sync_handler+0x13c/0x158
> [ 50.500422] el0t_64_sync+0x1a4/0x1a8
> [ 50.501378] BUG: scheduling while atomic: singlethread/102/0x00000000
> [ 50.517641] INFO: lockdep is turned off.
> [ 50.518206] Modules linked in:
> [ 50.521135] CPU: 2 PID: 102 Comm: singlethread Tainted: G W
> 6.9.0-rc2-00257-g2d9f63c285db #128
> [ 50.522026] Hardware name: linux,dummy-virt (DT)
> [ 50.522623] Call trace:
> [ 50.522993] dump_backtrace+0x9c/0x100
> [ 50.523527] show_stack+0x20/0x38
> [ 50.523950] dump_stack_lvl+0xec/0x150
> [ 50.524405] dump_stack+0x18/0x28
> [ 50.524849] __schedule_bug+0x80/0xe0
> [ 50.525309] __schedule+0xb1c/0xc00
> [ 50.525750] schedule+0x58/0x170
> [ 50.526227] schedule_preempt_disabled+0x2c/0x50
> [ 50.526762] rwsem_down_write_slowpath+0x1ac/0x718
> [ 50.527342] down_write+0xf8/0x1a8
> [ 50.527857] split_huge_page_to_list_to_order+0x3c8/0x1130
> [ 50.528437] madvise_cold_or_pageout_pte_range+0x84c/0xea0
> [ 50.529012] walk_pgd_range+0x388/0x7d8
> [ 50.529442] __walk_page_range+0x1e0/0x1f0
> [ 50.529896] walk_page_range+0x1f0/0x2c8
> [ 50.530342] madvise_pageout+0xf8/0x280
> [ 50.530878] madvise_vma_behavior+0x310/0x9b8
> [ 50.531395] madvise_walk_vmas+0xc0/0x128
> [ 50.531849] do_madvise.part.0+0xf8/0x518
> [ 50.532330] __arm64_sys_madvise+0x68/0x88
> [ 50.532829] invoke_syscall+0x50/0x128
> [ 50.533374] el0_svc_common.constprop.0+0x48/0xf8
> [ 50.533992] do_el0_svc+0x28/0x40
> [ 50.534498] el0_svc+0x50/0x150
> [ 50.535029] el0t_64_sync_handler+0x13c/0x158
> [ 50.535588] el0t_64_sync+0x1a4/0x1a8
>
>
>
>> include/linux/pgtable.h | 30 ++++++++++++++
>> mm/internal.h | 12 +++++-
>> mm/madvise.c | 88 ++++++++++++++++++++++++-----------------
>> mm/memory.c | 4 +-
>> 4 files changed, 93 insertions(+), 41 deletions(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index 8185939df1e8..391f56a1b188 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -361,6 +361,36 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>> }
>> #endif
>>
>> +#ifndef mkold_ptes
>> +/**
>> + * mkold_ptes - Mark PTEs that map consecutive pages of the same folio as old.
>> + * @vma: VMA the pages are mapped into.
>> + * @addr: Address the first page is mapped at.
>> + * @ptep: Page table pointer for the first entry.
>> + * @nr: Number of entries to mark old.
>> + *
>> + * May be overridden by the architecture; otherwise, implemented as a simple
>> + * loop over ptep_test_and_clear_young().
>> + *
>> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
>> + * some PTEs might be write-protected.
>> + *
>> + * Context: The caller holds the page table lock. The PTEs map consecutive
>> + * pages that belong to the same folio. The PTEs are all in the same PMD.
>> + */
>> +static inline void mkold_ptes(struct vm_area_struct *vma, unsigned long addr,
>> + pte_t *ptep, unsigned int nr)
>> +{
>> + for (;;) {
>> + ptep_test_and_clear_young(vma, addr, ptep);
>> + if (--nr == 0)
>> + break;
>> + ptep++;
>> + addr += PAGE_SIZE;
>> + }
>> +}
>> +#endif
>> +
>> #ifndef __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
>> #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG)
>> static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
>> diff --git a/mm/internal.h b/mm/internal.h
>> index eadb79c3a357..efee8e4cd2af 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -130,6 +130,8 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>> * @flags: Flags to modify the PTE batch semantics.
>> * @any_writable: Optional pointer to indicate whether any entry except the
>> * first one is writable.
>> + * @any_young: Optional pointer to indicate whether any entry except the
>> + * first one is young.
>> *
>> * Detect a PTE batch: consecutive (present) PTEs that map consecutive
>> * pages of the same large folio.
>> @@ -145,16 +147,18 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>> */
>> static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>> pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
>> - bool *any_writable)
>> + bool *any_writable, bool *any_young)
>> {
>> unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
>> const pte_t *end_ptep = start_ptep + max_nr;
>> pte_t expected_pte, *ptep;
>> - bool writable;
>> + bool writable, young;
>> int nr;
>>
>> if (any_writable)
>> *any_writable = false;
>> + if (any_young)
>> + *any_young = false;
>>
>> VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>> VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
>> @@ -168,6 +172,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>> pte = ptep_get(ptep);
>> if (any_writable)
>> writable = !!pte_write(pte);
>> + if (any_young)
>> + young = !!pte_young(pte);
>> pte = __pte_batch_clear_ignored(pte, flags);
>>
>> if (!pte_same(pte, expected_pte))
>> @@ -183,6 +189,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>>
>> if (any_writable)
>> *any_writable |= writable;
>> + if (any_young)
>> + *any_young |= young;
>>
>> nr = pte_batch_hint(ptep, pte);
>> expected_pte = pte_advance_pfn(expected_pte, nr);
>> diff --git a/mm/madvise.c b/mm/madvise.c
>> index 070bedb4996e..bd00b83e7c50 100644
>> --- a/mm/madvise.c
>> +++ b/mm/madvise.c
>> @@ -336,6 +336,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>> LIST_HEAD(folio_list);
>> bool pageout_anon_only_filter;
>> unsigned int batch_count = 0;
>> + int nr;
>>
>> if (fatal_signal_pending(current))
>> return -EINTR;
>> @@ -423,7 +424,8 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>> return 0;
>> flush_tlb_batched_pending(mm);
>> arch_enter_lazy_mmu_mode();
>> - for (; addr < end; pte++, addr += PAGE_SIZE) {
>> + for (; addr < end; pte += nr, addr += nr * PAGE_SIZE) {
>> + nr = 1;
>> ptent = ptep_get(pte);
>>
>> if (++batch_count == SWAP_CLUSTER_MAX) {
>> @@ -447,55 +449,67 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>> continue;
>>
>> /*
>> - * Creating a THP page is expensive so split it only if we
>> - * are sure it's worth. Split it if we are only owner.
>> + * If we encounter a large folio, only split it if it is not
>> + * fully mapped within the range we are operating on. Otherwise
>> + * leave it as is so that it can be swapped out whole. If we
>> + * fail to split a folio, leave it in place and advance to the
>> + * next pte in the range.
>> */
>> if (folio_test_large(folio)) {
>> - int err;
>> -
>> - if (folio_likely_mapped_shared(folio))
>> - break;
>> - if (pageout_anon_only_filter && !folio_test_anon(folio))
>> - break;
>> - if (!folio_trylock(folio))
>> - break;
>> - folio_get(folio);
>> - arch_leave_lazy_mmu_mode();
>> - pte_unmap_unlock(start_pte, ptl);
>> - start_pte = NULL;
>> - err = split_folio(folio);
>> - folio_unlock(folio);
>> - folio_put(folio);
>> - if (err)
>> - break;
>> - start_pte = pte =
>> - pte_offset_map_lock(mm, pmd, addr, &ptl);
>> - if (!start_pte)
>> - break;
>> - arch_enter_lazy_mmu_mode();
>> - pte--;
>> - addr -= PAGE_SIZE;
>> - continue;
>> + const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
>> + FPB_IGNORE_SOFT_DIRTY;
>> + int max_nr = (end - addr) / PAGE_SIZE;
>> + bool any_young;
>> +
>> + nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
>> + fpb_flags, NULL, &any_young);
>> + if (any_young)
>> + ptent = pte_mkyoung(ptent);
>> +
>> + if (nr < folio_nr_pages(folio)) {
>> + int err;
>> +
>> + if (folio_likely_mapped_shared(folio))
>> + continue;
>> + if (pageout_anon_only_filter && !folio_test_anon(folio))
>> + continue;
>> + if (!folio_trylock(folio))
>> + continue;
>> + folio_get(folio);
>> + arch_leave_lazy_mmu_mode();
>> + pte_unmap_unlock(start_pte, ptl);
>> + start_pte = NULL;
>> + err = split_folio(folio);
>> + folio_unlock(folio);
>> + folio_put(folio);
>> + if (err)
>> + continue;

The ptl is unlocked at this point. This used to break, but now it continues
without the lock held!

>> + start_pte = pte =
>> + pte_offset_map_lock(mm, pmd, addr, &ptl);
>> + if (!start_pte)
>> + break;

I think we would want to move the condition to here:

if (err)
continue;

I'll fix it in the next version.

Thanks,
Ryan

>> + arch_enter_lazy_mmu_mode();
>> + nr = 0;
>> + continue;
>> + }
>> }
>>
>> /*
>> * Do not interfere with other mappings of this folio and
>> - * non-LRU folio.
>> + * non-LRU folio. If we have a large folio at this point, we
>> + * know it is fully mapped so if its mapcount is the same as its
>> + * number of pages, it must be exclusive.
>> */
>> - if (!folio_test_lru(folio) || folio_mapcount(folio) != 1)
>> + if (!folio_test_lru(folio) ||
>> + folio_mapcount(folio) != folio_nr_pages(folio))
>> continue;
>>
>> if (pageout_anon_only_filter && !folio_test_anon(folio))
>> continue;
>>
>> - VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
>> -
>> if (!pageout && pte_young(ptent)) {
>> - ptent = ptep_get_and_clear_full(mm, addr, pte,
>> - tlb->fullmm);
>> - ptent = pte_mkold(ptent);
>> - set_pte_at(mm, addr, pte, ptent);
>> - tlb_remove_tlb_entry(tlb, pte, addr);
>> + mkold_ptes(vma, addr, pte, nr);
>> + tlb_remove_tlb_entries(tlb, pte, nr, addr);
>> }
>>
>> /*
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 9d844582ba38..b5b48f4cf2af 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -989,7 +989,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>> flags |= FPB_IGNORE_SOFT_DIRTY;
>>
>> nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags,
>> - &any_writable);
>> + &any_writable, NULL);
>> folio_ref_add(folio, nr);
>> if (folio_test_anon(folio)) {
>> if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
>> @@ -1553,7 +1553,7 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
>> */
>> if (unlikely(folio_test_large(folio) && max_nr != 1)) {
>> nr = folio_pte_batch(folio, addr, pte, ptent, max_nr, fpb_flags,
>> - NULL);
>> + NULL, NULL);
>>
>> zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, nr,
>> addr, details, rss, force_flush,
>> --
>> 2.25.1
>>

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