Re: [PATCH RESEND v15 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

On 5/22/23 3:24 PM, Muhammad Usama Anjum wrote:
> On 4/26/23 7:13 PM, Peter Xu wrote:
>> Hi, Muhammad,
>>
>> On Wed, Apr 26, 2023 at 12:06:23PM +0500, Muhammad Usama Anjum wrote:
>>> On 4/20/23 11:01 AM, Muhammad Usama Anjum wrote:
>>>> +/* Supported flags */
>>>> +#define PM_SCAN_OP_GET	(1 << 0)
>>>> +#define PM_SCAN_OP_WP	(1 << 1)
>>> We have only these flag options available in PAGEMAP_SCAN IOCTL.
>>> PM_SCAN_OP_GET must always be specified for this IOCTL. PM_SCAN_OP_WP can
>>> be specified as need. But PM_SCAN_OP_WP cannot be specified without
>>> PM_SCAN_OP_GET. (This was removed after you had asked me to not duplicate
>>> functionality which can be achieved by UFFDIO_WRITEPROTECT.)
>>>
>>> 1) PM_SCAN_OP_GET | PM_SCAN_OP_WP
>>> vs
>>> 2) UFFDIO_WRITEPROTECT
>>>
>>> After removing the usage of uffd_wp_range() from PAGEMAP_SCAN IOCTL, we are
>>> getting really good performance which is comparable just like we are
>>> depending on SOFT_DIRTY flags in the PTE. But when we want to perform wp,
>>> PM_SCAN_OP_GET | PM_SCAN_OP_WP is more desirable than UFFDIO_WRITEPROTECT
>>> performance and behavior wise.
>>>
>>> I've got the results from someone else that UFFDIO_WRITEPROTECT block
>>> pagefaults somehow which PAGEMAP_IOCTL doesn't. I still need to verify this
>>> as I don't have tests comparing them one-to-one.
>>>
>>> What are your thoughts about it? Have you thought about making
>>> UFFDIO_WRITEPROTECT perform better?
>>>
>>> I'm sorry to mention the word "performance" here. Actually we want better
>>> performance to emulate Windows syscall. That is why we are adding this
>>> functionality. So either we need to see what can be improved in
>>> UFFDIO_WRITEPROTECT or can I please add only PM_SCAN_OP_WP back in
>>> pagemap_ioctl?
>>
>> I'm fine if you want to add it back if it works for you.  Though before
>> that, could you remind me why there can be a difference on performance?
> I've looked at the code again and I think I've found something. Lets look
> at exact performance numbers:
> 
> I've run 2 different tests. In first test UFFDIO_WRITEPROTECT is being used
> for engaging WP. In second test PM_SCAN_OP_WP is being used. I've measured
> the average write time to the same memory which is being WP-ed and total
> time of execution of these APIs:
> 
> **avg write time:**
> | No of pages            | 2000 | 8192 | 100000 | 500000 |
> |------------------------|------|------|--------|--------|
> | UFFDIO_WRITEPROTECT    | 2200 | 2300 | 4100   | 4200   |
> | PM_SCAN_OP_WP          | 2000 | 2300 | 2500   | 2800   |
> 
> **Execution time measured in rdtsc:**
> | No of pages            | 2000 | 8192  | 100000 | 500000 |
> |------------------------|------|-------|--------|--------|
> | UFFDIO_WRITEPROTECT    | 3200 | 14000 | 59000  | 58000  |
> | PM_SCAN_OP_WP          | 1900 | 7000  | 38000  | 40000  |
> 
> Avg write time for UFFDIO_WRITEPROTECT is 1.3 times slow. The execution
> time is 1.5 times slower in the case of UFFDIO_WRITEPROTECT. So
> UFFDIO_WRITEPROTECT is making writes slower to the pages and execution time
> is also slower.
> 
> This proves that PM_SCAN_OP_WP is better than UFFDIO_WRITEPROTECT. Although
> PM_SCAN_OP_WP and UFFDIO_WRITEPROTECT have been implemented differently. We
> should have seen no difference in performance. But we have quite a lot of
> difference in performance here. PM_SCAN_OP_WP takes read mm lock, uses
> walk_page_range() to walk over pages which finds VMAs from address ranges
> to walk over them and pagemap_scan_pmd_entry() is handling most of the work
> including tlb flushing. UFFDIO_WRITEPROTECT is also taking the mm lock and
> iterating from all the different page directories until a pte is found and
> then flags are updated there and tlb is flushed for every pte.
> 
> My next deduction would be that we are getting worse performance as we are
> flushing tlb for one page at a time in case of UFFDIO_WRITEPROTECT. While
> we flush tlb for 512 pages (moslty) at a time in case of PM_SCAN_OP_WP.
> I've just verified this by adding some logs to the change_pte_range() and
> pagemap_scan_pmd_entry(). Logs are attached. I've allocated memory of 1000
> pages and write-protected it with UFFDIO_WRITEPROTECT and PM_SCAN_OP_WP.
> The logs show that UFFDIO_WRITEPROTECT has flushed tlb 1000 times of size 1
> page each time. While PM_SCAN_OP_WP has flushed only 3 times of bigger
> sizes. I've learned over my last experience that tlb flush is very
> expensive. Probably this is what we need to improve if we don't want to add
> PM_SCAN_OP_WP?
> 
> The UFFDIO_WRITEPROTECT uses change_pte_range() which is very generic
> function and I'm not sure if can try to not do tlb flushes if uffd_wp is
> true. We can try to do flush somewhere else and hopefully we should do only
> one flush if possible. It will not be so straight forward to move away from
> generic fundtion. Thoughts?
I've just tested this theory of not doing per pte flushes and only did one
flush on entire range in uffd_wp_range(). But it didn't improve the
situation either. I was wrong that tlb flushes may be the cause.


> 
> 
>> Thanks,
>>
> 

-- 
BR,
Muhammad Usama Anjum