Re: [PATCH v5 4/8] mm: Add write-protect and clean utilities for address space ranges

["Thomas Hellström (VMware)" <thomas_os@shipmail.org>]

On 10/10/19 4:17 PM, Peter Zijlstra wrote:
> On Thu, Oct 10, 2019 at 03:24:47PM +0200, Thomas Hellström (VMware) wrote:
>> On 10/10/19 3:05 PM, Peter Zijlstra wrote:
>>> On Thu, Oct 10, 2019 at 02:43:10PM +0200, Thomas Hellström (VMware) wrote:
>>>> +/**
>>>> + * wp_shared_mapping_range - Write-protect all ptes in an address space range
>>>> + * @mapping: The address_space we want to write protect
>>>> + * @first_index: The first page offset in the range
>>>> + * @nr: Number of incremental page offsets to cover
>>>> + *
>>>> + * Note: This function currently skips transhuge page-table entries, since
>>>> + * it's intended for dirty-tracking on the PTE level. It will warn on
>>>> + * encountering transhuge write-enabled entries, though, and can easily be
>>>> + * extended to handle them as well.
>>>> + *
>>>> + * Return: The number of ptes actually write-protected. Note that
>>>> + * already write-protected ptes are not counted.
>>>> + */
>>>> +unsigned long wp_shared_mapping_range(struct address_space *mapping,
>>>> +				      pgoff_t first_index, pgoff_t nr)
>>>> +{
>>>> +	struct wp_walk wpwalk = { .total = 0 };
>>>> +
>>>> +	i_mmap_lock_read(mapping);
>>>> +	WARN_ON(walk_page_mapping(mapping, first_index, nr, &wp_walk_ops,
>>>> +				  &wpwalk));
>>>> +	i_mmap_unlock_read(mapping);
>>>> +
>>>> +	return wpwalk.total;
>>>> +}
>>> That's a read lock, this means there's concurrency to self. What happens
>>> if someone does two concurrent wp_shared_mapping_range() on the same
>>> mapping?
>>>
>>> The thing is, because of pte_wrprotect() the iteration that starts last
>>> will see a smaller pte_write range, if it completes first and does
>>> flush_tlb_range(), it will only flush a partial range.
>>>
>>> This is exactly what {inc,dec}_tlb_flush_pending() is for, but you're
>>> not using mm_tlb_flush_nested() to detect the situation and do a bigger
>>> flush.
>>>
>>> Or if you're not needing that, then I'm missing why.
>> Good catch. Thanks,
>>
>> Yes the read lock is not intended to protect against concurrent users but to
>> protect the vmas from disappearing under us. Since it fundamentally makes no
>> sense having two concurrent threads picking up dirty ptes on the same
>> address_space range we have an external range-based lock to protect against
>> that.
> Nothing mandates/verifies the function you expose is used exclusively.
> Therefore you cannot make assumptions on that range lock your user has.
>
>> However, that external lock doesn't protect other code  from concurrently
>> modifying ptes and having the mm's  tlb_flush_pending increased, so I guess
>> we unconditionally need to test for that and do a full range flush if
>> necessary?
> Yes, something like:
>
> 	if (mm_tlb_flush_nested(mm))
> 		flush_tlb_range(walk->vma, walk->vma->vm_start, walk->vma->vm_end);
> 	else  if (wpwalk->tlbflush_end > wpwalk->tlbflush_start)
> 		flush_tlb_range(walk->vma, wpwalk->tlbflush_start, wpwalk->tlbflush_end);
>
Hi, Peter,

I've updated the patch to incorporate something similar to the above. 
Since you've looked at the patch, any chance of an R-B?

Thanks,

Thomas