Re: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning

From: Muhammad Usama Anjum <usama.anjum@collabora.com>
To: "Michał Mirosław" <emmir@google.com>
Cc: "Muhammad Usama Anjum" <usama.anjum@collabora.com>,
	"Michał Mirosław" <mirq-linux@rere.qmqm.pl>,
	"Andrei Vagin" <avagin@gmail.com>,
	"Danylo Mocherniuk" <mdanylo@google.com>,
	"Alex Sierra" <alex.sierra@amd.com>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Axel Rasmussen" <axelrasmussen@google.com>,
	"Christian Brauner" <brauner@kernel.org>,
	"Cyrill Gorcunov" <gorcunov@gmail.com>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Greg KH" <gregkh@linuxfoundation.org>,
	"Gustavo A . R . Silva" <gustavoars@kernel.org>,
	"Liam R . Howlett" <Liam.Howlett@oracle.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Mike Rapoport" <rppt@kernel.org>,
	"Nadav Amit" <namit@vmware.com>,
	"Pasha Tatashin" <pasha.tatashin@soleen.com>,
	"Paul Gofman" <pgofman@codeweavers.com>,
	"Peter Xu" <peterx@redhat.com>, "Shuah Khan" <shuah@kernel.org>,
	"Suren Baghdasaryan" <surenb@google.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Yang Shi" <shy828301@gmail.com>,
	"Yun Zhou" <yun.zhou@windriver.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
	kernel@collabora.com
Subject: Re: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning
Date: Tue, 25 Jul 2023 14:09:32 +0500	[thread overview]
Message-ID: <1afedab8-5929-61e5-b0da-9c70dc01c254@collabora.com> (raw)
In-Reply-To: <e1ead2e8-046a-31d9-8df9-27cdd7b7ff83@collabora.com>

On 7/25/23 12:23 PM, Muhammad Usama Anjum wrote:
> On 7/24/23 9:10 PM, Michał Mirosław wrote:
> [...]>>>> @@ -2219,22 +2225,24 @@ static int pagemap_scan_get_args(struct
> pm_scan_arg
>>>>> *arg,
>>>>>              arg->category_anyof_mask | arg->return_mask) & ~PM_SCAN_CATEGORIES)
>>>>>                 return -EINVAL;
>>>>>
>>>>> -       start = untagged_addr((unsigned long)arg->start);
>>>>> -       end = untagged_addr((unsigned long)arg->end);
>>>>> -       vec = untagged_addr((unsigned long)arg->vec);
>>>>> +       arg->start = untagged_addr((unsigned long)arg->start);
>>>>> +       arg->end = untagged_addr((unsigned long)arg->end);
>>>>> +       arg->vec = untagged_addr((unsigned long)arg->vec);
>>>>
>>>> BTW, We should we keep the tag in args writeback().
>>> Sorry what?
>>> After this function, the start, end and vec would be used. We need to make
>>> sure that the address are untagged before that.
>>
>> We do write back the address the walk ended at to arg->start in
>> userspace. This pointer I think needs the tag reconstructed so that
>> retrying the ioctl() will be possible.
> Even if we reconstruct the tag for end and vec, We need to update the start
> address. Can we just put same tag as original start in it? I'm not sure.
The special users would use tags. If they are using it, they'll just re-add
the tag in next invocation. I think this implementation is correct.

> 

-- 
BR,
Muhammad Usama Anjum