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: <b737dceb-a228-7ffe-0758-421505f1a61d@collabora.com>
Date:   Fri, 7 Apr 2023 02:12:31 +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 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)
>> +{
>> +       struct pagemap_scan_private *p = walk->private;
>> +       bool is_written, is_file, is_present, is_swap;
>> +       struct vm_area_struct *vma = walk->vma;
>> +       unsigned long addr = end;
>> +       spinlock_t *ptl;
>> +       int ret = 0;
>> +       pte_t *pte;
>> +
>> +#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?


--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1957,11 +1957,11 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
unsigned long start,

 		return ret;
 	}
-process_smaller_pages:
+
 	if (pmd_trans_unstable(pmd))
 		return 0;
 #endif
-
+process_smaller_pages:
 	for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE)


-- 
BR,
Muhammad Usama Anjum