Re: [Devel] [PATCH vz10 12/24] oracle/mm: enter COW seqcount/mmu-notifier section in copy_page_range_mt()

[Konstantin Khorenko <khorenko@virtuozzo.com>]

i'm dropping this patch.
The bug is fixed by another patch, taken from Oracle UEK8 kernel:

[PATCH hci-8.1 1/8] oracle/mm: fix VM_BUG_ON hit when preserving memory

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 7/24/26 15:09, Pavel Tikhomirov wrote:
> Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> 
> This is really non-obvious, the logic behind "mirroring" is sound, also
> we should report this to oracle, I don't see full miroring there, they have
> raw_write_seqcount_begin(), but not mmu_notifier_...
> 
> On 7/6/26 12:59, Konstantin Khorenko wrote:
>> copy_page_range_mt() is the padata-multithreaded copy used by vma_dup()
>> to preserve a VM_EXEC_KEEP (MADV_DOEXEC) VMA across exec. A MADV_DOEXEC
>> VMA is private anon, so is_cow_mapping() is true and the copy write
>> protects the source PTEs - a permission downgrade. Unlike the normal
>> fork path copy_page_range(), this helper jumped straight into
>> padata_do_multithreaded() without the COW prologue/epilogue, so it:
>>
>>   - never entered src_mm->write_protect_seq (left it even), which trips
>>     the DEBUG_VM invariant in folio_needs_cow_for_dma() reached from
>>     folio_try_dup_anon_rmap_pte() during the copy:
>>
>>       kernel BUG at include/linux/mm.h:2054!
>>       copy_present_ptes <- copy_pte_range <- copy_page_range_chunk
>>         <- padata_do_multithreaded <- copy_page_range_mt <- vma_dup
>>         <- exec_mmap <- begin_new_exec <- load_elf_binary
>>
>>   - never called mmu_notifier_invalidate_range_start/end(), a real latent
>>     bug: a secondary MMU (KVM, IOMMU SVA, GPU) registered on the old mm
>>     outlives de_thread() and could keep writable mappings of pages whose
>>     PTEs the exec copy just wrprotected and shared into the new mm.
>>
>> Mirror copy_page_range(): for a COW mapping, invalidate secondary MMUs
>> and take the write_protect_seq write side around the padata job. The
>> exec'ing task is single-threaded by now and is the only user of src_mm,
>> so there is no concurrent seqcount writer (fork needs mmap_write_lock)
>> or GUP-fast reader; taking the raw write side while holding only
>> mmap_read_lock is safe, which is why vma_assert_write_locked() from
>> copy_page_range() is intentionally not carried over here.
>>
>> Fixes: 11e4b1dc50e3 ("oracle/mm: use padata for copying page ranges in vma_dup()")
>> Feature: oracle/mm: MADV_DOEXEC madvise() flag
>> https://virtuozzo.atlassian.net/browse/VSTOR-137234
>> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
>> ---
>>  mm/memory.c | 35 ++++++++++++++++++++++++++++++++++-
>>  1 file changed, 34 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 837194e87664..4d04591f8f40 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1478,6 +1478,10 @@ static int copy_page_range_chunk(unsigned long addr,
>>  int copy_page_range_mt(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>>  {
>>  	struct copy_page_range_args args = { dst_vma, src_vma };
>> +	struct mm_struct *src_mm = src_vma->vm_mm;
>> +	struct mmu_notifier_range range;
>> +	bool is_cow;
>> +	int ret;
>>  	struct padata_mt_job job = {
>>  		.thread_fn   = copy_page_range_chunk,
>>  		.fn_arg      = &args,
>> @@ -1490,7 +1494,36 @@ int copy_page_range_mt(struct vm_area_struct *dst_vma, struct vm_area_struct *sr
>>  
>>  	BUG_ON(!(src_vma->vm_flags & VM_EXEC_KEEP));
>>  
>> -	return padata_do_multithreaded(&job);
>> +	/*
>> +	 * Mirror copy_page_range(): for a COW mapping the copy write
>> +	 * protects the source PTEs, which is a permission downgrade, so
>> +	 * secondary MMUs must be invalidated, and write_protect_seq must be
>> +	 * entered (odd) so GUP-fast cannot race with the wrprotect
>> +	 * (folio_needs_cow_for_dma() asserts this under CONFIG_DEBUG_VM).
>> +	 *
>> +	 * The exec'ing task is single-threaded by now and the only user of
>> +	 * src_mm, so no concurrent write_protect_seq writer (fork, which
>> +	 * needs mmap_write_lock) or GUP-fast reader can exist; taking the
>> +	 * raw write side while holding only mmap_read_lock is safe here, and
>> +	 * the padata helper threads only read the seqcount.
>> +	 */
>> +	is_cow = is_cow_mapping(src_vma->vm_flags);
>> +	if (is_cow) {
>> +		mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
>> +					0, src_mm, src_vma->vm_start,
>> +					src_vma->vm_end);
>> +		mmu_notifier_invalidate_range_start(&range);
>> +		raw_write_seqcount_begin(&src_mm->write_protect_seq);
>> +	}
>> +
>> +	ret = padata_do_multithreaded(&job);
>> +
>> +	if (is_cow) {
>> +		raw_write_seqcount_end(&src_mm->write_protect_seq);
>> +		mmu_notifier_invalidate_range_end(&range);
>> +	}
>> +
>> +	return ret;
>>  }
>>  #endif /* CONFIG_PADATA */
>>  
> 

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