Date:   Tue, 23 Nov 2021 20:51:21 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Mina Almasry <almasrymina@google.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        David Hildenbrand <david@redhat.com>,
        "Paul E . McKenney" <paulmckrcu@fb.com>,
        Yu Zhao <yuzhao@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>,
        Ivan Teterevkov <ivan.teterevkov@nutanix.com>,
        Florian Schmidt <florian.schmidt@nutanix.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap
Message-ID: <YZ1USY+zB1PP24Z1@casper.infradead.org>
References: <20211123000102.4052105-1-almasrymina@google.com>
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On Mon, Nov 22, 2021 at 04:01:02PM -0800, Mina Almasry wrote:
> Add PM_THP_MAPPED MAPPING to allow userspace to detect whether a given virt
> address is currently mapped by a transparent huge page or not.  Example
> use case is a process requesting THPs from the kernel (via a huge tmpfs
> mount for example), for a performance critical region of memory.  The
> userspace may want to query whether the kernel is actually backing this
> memory by hugepages or not.

So you want this bit to be clear if the memory is backed by a hugetlb
page?

>  		if (page && page_mapcount(page) == 1)
>  			flags |= PM_MMAP_EXCLUSIVE;
> +		if (page && is_transparent_hugepage(page))
> +			flags |= PM_THP_MAPPED;

because honestly i'd expect it to be more useful to mean "This memory
is mapped by a PMD entry" and then the code would look like:

		if (page)
			flags |= PM_PMD_MAPPED;

(and put a corresponding change in pagemap_hugetlb_range)