linux-kernel - Re: [PATCH v32 2/6] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Message-ID: <35fe8ddc-cbf3-403c-a40a-2e7b6842ec98@collabora.com>
Date:   Mon, 21 Aug 2023 19:11:15 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Peter Xu <peterx@...hat.com>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.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,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v32 2/6] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On 8/19/23 6:04 AM, Michał Mirosław wrote:
> On Wed, Aug 16, 2023 at 04:30:45PM +0500, Muhammad Usama Anjum wrote:
>> The PAGEMAP_SCAN IOCTL on the pagemap file can be used to get or optionally
>> clear the info about page table entries.
> [...]
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
> [...]
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +static unsigned long pagemap_thp_category(pmd_t pmd)
>> +{
>> +	unsigned long categories = PAGE_IS_HUGE;
>> +
>> +	/*
>> +	 * THPs don't support file-backed memory. So PAGE_IS_FILE
>> +	 * is not checked here.
>> +	 */
> 
> It seems that we can have THP for files: ref. recent LKML thread [1].
> 
> [1] https://lkml.org/lkml/2023/8/16/1212
I didn't know about this. It seems like khugepaged can create file THPs.
Not sure if there is direct way to create file backed THPs. Maybe they'll
add this support after adding writing support to these file backed THPs.
READ_ONLY_THP_FOR_FS mentions this.

I can add a few lines of code for this here.

> 
>> +	if (pmd_present(pmd)) {
>> +		categories |= PAGE_IS_PRESENT;
>> +		if (!pmd_uffd_wp(pmd))
>> +			categories |= PAGE_IS_WRITTEN;
>> +		if (is_zero_pfn(pmd_pfn(pmd)))
>> +			categories |= PAGE_IS_PFNZERO;
>> +	} else if (is_swap_pmd(pmd)) {
>> +		categories |= PAGE_IS_SWAPPED;
>> +		if (!pmd_swp_uffd_wp(pmd))
>> +			categories |= PAGE_IS_WRITTEN;
>> +	}
>> +
>> +	return categories;
>> +}
> 
> Best Regards
> Michał Mirosław

-- 
BR,
Muhammad Usama Anjum