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

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Hi Peter,

Thank you so much for reviewing!

On 2/9/23 2:12 AM, Peter Xu wrote:
> 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@redhat.com>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.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..
Will fix in next version.

> 
>> -		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.
Thank you so much! This wouldn't have been possible without your help.

> 

-- 
BR,
Muhammad Usama Anjum