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

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Date:   Wed, 26 Apr 2023 10:13:27 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Paul Gofman <pgofman@...eweavers.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Michał Mirosław <emmir@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH RESEND v15 2/5] fs/proc/task_mmu: Implement IOCTL to get
 and optionally clear info about PTEs

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