linux-kernel - Re: [PATCH v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Message-ID: <b3e4d688-b96f-7c44-a6be-375d44263c85@collabora.com>
Date:   Fri, 7 Apr 2023 15:14:47 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Michał Mirosław <emmir@...gle.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.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>,
        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 v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On 4/7/23 3:04 PM, Michał Mirosław wrote:
> On Fri, 7 Apr 2023 at 11:35, Muhammad Usama Anjum
> <usama.anjum@...labora.com> wrote:
>> On 4/7/23 12:23 PM, Michał Mirosław wrote:
>>> On Thu, 6 Apr 2023 at 23:12, Muhammad Usama Anjum
>>> <usama.anjum@...labora.com> wrote:
>>>> On 4/7/23 1:12 AM, Michał Mirosław wrote:
>>>>> On Thu, 6 Apr 2023 at 09:40, Muhammad Usama Anjum
>>>>> <usama.anjum@...labora.com> wrote:
>>>>> [...]
>>>>>> --- a/fs/proc/task_mmu.c
>>>>>> +++ b/fs/proc/task_mmu.c
>>>>> [...]
>>>>>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>>>>>> +                                 unsigned long end, struct mm_walk *walk)
>>>>>> +{
>>> [...]
>>>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>>> +       ptl = pmd_trans_huge_lock(pmd, vma);
>>>>>> +       if (ptl) {
>>>>> [...]
>>>>>> +               return ret;
>>>>>> +       }
>>>>>> +process_smaller_pages:
>>>>>> +       if (pmd_trans_unstable(pmd))
>>>>>> +               return 0;
>>>>>
>>>>> Why pmd_trans_unstable() is needed here and not only after split_huge_pmd()?
>>>> I'm not entirely sure. But the idea is if THP is unstable, we should
>>>> return. As it doesn't seem like after splitting THP can be unstable, we
>>>> should not check it. Do you agree with the following?
>>>
>>> The description of pmd_trans_unstable() [1] seems to indicate that it
>>> is needed only after split_huge_pmd().
>>>
>>> [1] https://elixir.bootlin.com/linux/v6.3-rc5/source/include/linux/pgtable.h#L1394
>> Sorry, yeah pmd_trans_unstable() is need after split. But it is also needed
>> in normal case when ptl is NULL to rule out the case if pmd is unstable
>> before performing operation on normal pages:
>>
>> ptl = pmd_trans_huge_lock(pmd, vma);
>> if (ptl) {
>> ...
>> }
>> if (pmd_trans_unstable(pmd))
>>         return 0;
>>
>> This file has usage examples of pmd_trans_unstable():
>>
>> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L634
>> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1195
>> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1543
>> https://elixir.bootlin.com/linux/v6.3-rc5/source/fs/proc/task_mmu.c#L1887
>>
>> So we are good with what we have in this patch.
> 
> Shouldn't we signal ACTION_AGAIN then in order to call .pte_hole?
I'm not sure. I've not done research on it if we need to signal
ACTION_AGAIN as this function pagemap_scan_pmd_entry() mimics how
pagemap_pmd_range() handles reads to the pagemap file. pagemap_pmd_range()
isn't doing anything if pmd is unstable. Hence we also not doing anything.

> 
> Best Regards
> Michał Mirosław

-- 
BR,
Muhammad Usama Anjum