linux-kernel - Re: [PATCH v10 1/6] userfaultfd: Add UFFD WP Async support

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Date:   Wed, 8 Feb 2023 16:12:12 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.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>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v10 1/6] userfaultfd: Add UFFD WP Async support

On Thu, Feb 02, 2023 at 04:29:10PM +0500, Muhammad Usama Anjum wrote:
> Add new WP Async mode (UFFD_FEATURE_WP_ASYNC) which resolves the page
> faults on its own. It can be used to track that which pages have been
> written-to from the time the pages were write-protected. It is very
> efficient way to track the changes as uffd is by nature pte/pmd based.
> 
> UFFD synchronous WP sends the page faults to the userspace where the
> pages which have been written-to can be tracked. But it is not efficient.
> This is why this asynchronous version is being added. After setting the
> WP Async, the pages which have been written to can be found in the pagemap
> file or information can be obtained from the PAGEMAP_IOCTL.
> 
> Suggested-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> ---
> Changes in v10:
> - Build fix
> - Update comments and add error condition to return error from uffd
>   register if hugetlb pages are present when wp async flag is set
> 
> Changes in v9:
> - Correct the fault resolution with code contributed by Peter
> 
> Changes in v7:
> - Remove UFFDIO_WRITEPROTECT_MODE_ASYNC_WP and add UFFD_FEATURE_WP_ASYNC
> - Handle automatic page fault resolution in better way (thanks to Peter)
> 
> update to wp async
> 
> uffd wp async
> ---
>  fs/userfaultfd.c                 | 20 ++++++++++++++++++--
>  include/linux/userfaultfd_k.h    | 11 +++++++++++
>  include/uapi/linux/userfaultfd.h | 10 +++++++++-
>  mm/memory.c                      | 23 ++++++++++++++++++++---
>  4 files changed, 58 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 15a5bf765d43..422f2530c63e 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1422,10 +1422,15 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>  			goto out_unlock;
>  
>  		/*
> -		 * Note vmas containing huge pages
> +		 * Note vmas containing huge pages. Hugetlb isn't supported
> +		 * with UFFD_FEATURE_WP_ASYNC.
>  		 */

Need to set "ret = -EINVAL;" here.  Or..

> -		if (is_vm_hugetlb_page(cur))
> +		if (is_vm_hugetlb_page(cur)) {
> +			if (ctx->features & UFFD_FEATURE_WP_ASYNC)
> +				goto out_unlock;

.. it'll return -EBUSY, which does not sound like the right errcode here.

> +

Drop this empty line?

>  			basic_ioctls = true;
> +		}
>  
>  		found = true;
>  	}

Other than that looks good, thanks.

-- 
Peter Xu