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

[Peter Xu <peterx@redhat.com>]

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?

Thanks,

-- 
Peter Xu