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

[Muhammad Usama Anjum]

Hi Peter,

Thank you for your reply.

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?
The only difference can be that UFFDIO_WRITEPROTECT acquires read mm lock
once for entire duration. But for PAGEMAP_SCAN IOCTL, we acquire and
release for each PMD to keep intermediate buffer short.

This must be hard to convince you. So I'll write some test to see what is
the exact difference and show you the numbers.

> 
> Thanks,
> 

-- 
BR,
Muhammad Usama Anjum