Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 26 Apr 2023 10:13:27 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     Paul Gofman <pgofman@codeweavers.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Shuah Khan <shuah@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Yang Shi <shy828301@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Yun Zhou <yun.zhou@windriver.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <emmir@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Andrei Vagin <avagin@gmail.com>,
        Alex Sierra <alex.sierra@amd.com>,
        Matthew Wilcox <willy@infradead.org>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Danylo Mocherniuk <mdanylo@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "Gustavo A . R . Silva" <gustavoars@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        linux-kernel@vger.kernel.org, Mike Rapoport <rppt@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kselftest@vger.kernel.org,
        Greg KH <gregkh@linuxfoundation.org>, kernel@collabora.com,
        Nadav Amit <namit@vmware.com>
Subject: Re: [PATCH RESEND v15 2/5] fs/proc/task_mmu: Implement IOCTL to get
 and optionally clear info about PTEs
Message-ID: <ZEkxh6dbnAOuYuJj@x1n>
References: <20230420060156.895881-1-usama.anjum@collabora.com>
 <20230420060156.895881-3-usama.anjum@collabora.com>
 <fd9ddd43-6737-88bd-4054-3d5b94534271@collabora.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <fd9ddd43-6737-88bd-4054-3d5b94534271@collabora.com>
Precedence: bulk

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