Date:   Tue, 16 Nov 2021 09:59:10 +0800
From:   Peter Xu <peterx@redhat.com>
To:     Mina Almasry <almasrymina@google.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Paul E . McKenney" <paulmckrcu@fb.com>,
        Yu Zhao <yuzhao@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        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
Subject: Re: [PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap
Message-ID: <YZMQbiV9JQWd0EM+@xz-m1.local>
References: <20211107235754.1395488-1-almasrymina@google.com>
 <YYtuqsnOSxA44AUX@t490s>
 <CAHS8izP9zJYfqmDouA1otnD-CsQtWJSta0KhOQq81qLSTOHB4Q@mail.gmail.com>
 <YY4bFPkfUhlpUqvo@xz-m1.local>
 <CAHS8izP7_BBH9NGz3XoL2=xVniH6REor=biqDSZ4wR=NaFS-8A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <CAHS8izP7_BBH9NGz3XoL2=xVniH6REor=biqDSZ4wR=NaFS-8A@mail.gmail.com>
Precedence: bulk

On Mon, Nov 15, 2021 at 02:50:26PM -0800, Mina Almasry wrote:
> PM_THP_MAPPED sounds good to me.
> 
> TBH I think I still prefer this approach because it's a very simple 2
> line patch which addresses the concrete use case I have well. I'm not
> too familiar with the smaps code to be honest but I think adding a
> range-based smaps API will be a sizeable patch to add a syscall,
> handle a stable interface, and handle cases where the memory range
> doesn't match a VMA boundary. I'm not sure the performance benefit
> would justify this patch and I'm not sure the extra info from smaps
> would be widely useful. However if you insist and folks believe this
> is the better approach I can prototype a range-based smaps and test
> its performance to see if it works for us as well, just let me know
> what kind of API you're envisioning.

Yeah indeed I haven't yet thought enough on such a new interface, it's just
that I think it'll be something that solves a broader range of requests
including the thp-aware issue, so I raised it up.

That shouldn't require a lot code change either afaiu, as smap_gather_stats()
already takes a "start" and I think what's missing is another end where we just
pass in 0 when we want the default vma->vm_end as the end of range.

I don't have a solid clue on other use case to ask for that more generic
interface, so please feel free to move on with it.  If you'll need a repost to
address the comment from Andrew on removing the debugging lines, please also
consider using the shorter PM_THP_MAPPED then it looks good to me too.

Thanks!

-- 
Peter Xu