From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 89BB7C433EF for ; Tue, 23 Nov 2021 23:17:06 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 916BB6B0073; Tue, 23 Nov 2021 18:16:50 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 89EDB6B0074; Tue, 23 Nov 2021 18:16:50 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6F1E76B0075; Tue, 23 Nov 2021 18:16:50 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0069.hostedemail.com [216.40.44.69]) by kanga.kvack.org (Postfix) with ESMTP id 588F06B0073 for ; Tue, 23 Nov 2021 18:16:50 -0500 (EST) Received: from smtpin28.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 219931849B252 for ; Tue, 23 Nov 2021 23:16:40 +0000 (UTC) X-FDA: 78841756518.28.E7F6458 Received: from mail-io1-f52.google.com (mail-io1-f52.google.com [209.85.166.52]) by imf04.hostedemail.com (Postfix) with ESMTP id 7BEA55000318 for ; Tue, 23 Nov 2021 23:16:36 +0000 (UTC) Received: by mail-io1-f52.google.com with SMTP id y16so751075ioc.8 for ; Tue, 23 Nov 2021 15:16:39 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=kM7gnTny43QDnYFYBUsykzKb/YIDf6V4JPjjkny8Fbs=; b=HUEdP7dL6RvpMRAWRK31o46crI1NcKJ5Kj1jT0kaYbbM9c5dVCR7RZ9Qgd9jcm43AY HtwBwSJZkEsD3opdhNfK1f9FQ4g1wiPy+nYo8j6yN3AV/p4hXM81mVH1h6+wyk5eznRl LqR78t6Si+RsE5nVyfu6wlDIenfPqTXThBMeNxBFuhTr7S4cHkTH6idM9eFTtsWiDTCs QDdeXV5+pIJJeAd2By0mmZTB0veqRUuezOAkZ6AzzRmh+s3pLdsv++E9lyLiw6INnbXx q1HcKxZ0hCXgZ76NmHn5tYCEoXUixiOziEFw44+sHXy6Q4cwBbj+JeuKyiaOegwwKhmp VvJQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=kM7gnTny43QDnYFYBUsykzKb/YIDf6V4JPjjkny8Fbs=; b=dIGgvJK5Te/9tBiO+4xWuVTGrBC/3LWBFSmJrr9HeWOyGoJksb0pidwDJcw66CwORH oDQEvCXDbRhxiaiUpNzQui1XCle6GPCbfH+qswJMii6D9Tj4GQe+QJyoNxuReF9wnu77 QU086IK9DnfekZHdN7rs/e59IE3ZsVRhZ0hS/L+dLlTbErftMk0aTuqZ3dowrchohj/T EDbR93PKZ1Qb1FZy10LI4ymGH8ya6P5H/yzK58UAiFVqJrHkEXKbc3+qgycf9eL8St5D UVIMokDbBdTP4Bhek8nAdivnafYdgA8pulWItmzuS1/7lhc30INleT61ctLN50ubaszO g6gQ== X-Gm-Message-State: AOAM5323RfVpf50ASQb5eWEGdeqO+5/CtfqUbljorwQeHGKCdrtDWOm5 pqGVWoeah9zfJOKmaNXQ2Bd4f8Vw/5hKAU3tfPIp+w== X-Google-Smtp-Source: ABdhPJy0JvkGEkbgZd3LhBD7aoiqSzR1lSBUaqfr5jk23IXyeyriLpIRI4TsLxjKV6oOvM0nh9GgzPPSX5zCwMWICAQ= X-Received: by 2002:a05:6602:2c85:: with SMTP id i5mr9498544iow.89.1637709398903; Tue, 23 Nov 2021 15:16:38 -0800 (PST) MIME-Version: 1.0 References: <20211123000102.4052105-1-almasrymina@google.com> In-Reply-To: From: Mina Almasry Date: Tue, 23 Nov 2021 15:16:27 -0800 Message-ID: Subject: Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap To: Matthew Wilcox Cc: Jonathan Corbet , David Hildenbrand , "Paul E . McKenney" , Yu Zhao , Andrew Morton , Peter Xu , Ivan Teterevkov , Florian Schmidt , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 7BEA55000318 X-Stat-Signature: 11sgq49d33kzfz3ugi61kazxgmprdw8d Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=HUEdP7dL; spf=pass (imf04.hostedemail.com: domain of almasrymina@google.com designates 209.85.166.52 as permitted sender) smtp.mailfrom=almasrymina@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1637709396-106838 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Nov 23, 2021 at 2:59 PM Matthew Wilcox wrote: > > On Tue, Nov 23, 2021 at 02:23:23PM -0800, Mina Almasry wrote: > > On Tue, Nov 23, 2021 at 2:03 PM Matthew Wilcox wrote: > > > > > > On Tue, Nov 23, 2021 at 01:47:33PM -0800, Mina Almasry wrote: > > > > On Tue, Nov 23, 2021 at 1:30 PM Matthew Wilcox wrote: > > > > > What I've been trying to communicate over the N reviews of this > > > > > patch series is that *the same thing is about to happen to THPs*. > > > > > Only more so. THPs are going to be of arbitrary power-of-two size, not > > > > > necessarily sizes supported by the hardware. That means that we need to > > > > > be extremely precise about what we mean by "is this a THP?" Do we just > > > > > mean "This is a compound page?" Do we mean "this is mapped by a PMD?" > > > > > Or do we mean something else? And I feel like I haven't been able to > > > > > get that information out of you. > > > > > > > > Yes, I'm very sorry for the trouble, but I'm also confused what the > > > > disconnect is. To allocate hugepages I can do like so: > > > > > > > > mount -t tmpfs -o huge=always tmpfs /mnt/mytmpfs > > > > > > > > or > > > > > > > > madvise(..., MADV_HUGEPAGE) > > > > > > > > Note I don't ask the kernel for a specific size, or a specific mapping > > > > mechanism (PMD/contig PTE/contig PMD/PUD), I just ask the kernel for > > > > 'huge' pages. I would like to know whether the kernel was successful > > > > in allocating a hugepage or not. Today a THP hugepage AFAICT is PMD > > > > mapped + is_transparent_hugepage(), which is the check I have here. In > > > > the future, THP may become an arbitrary power of two size, and I think > > > > I'll need to update this querying interface once/if that gets merged > > > > to the kernel. I.e, if in the future I allocate pages by using: > > > > > > > > mount -t tmpfs -o huge=2MB tmpfs /mnt/mytmpfs > > > > > > > > I need the kernel to tell me whether the mapping is 2MB size or not. > > > > > > > > If I allocate pages by using: > > > > > > > > mount -t tmpfs -o huge=pmd tmpfs /mnt/mytmps, > > > > > > > > Then I need the kernel to tell me whether the pages are PMD mapped or > > > > not, as I'm doing here. > > > > > > > > The current implementation is based on what the current THP > > > > implementation is in the kernel, and depending on future changes to > > > > THP I may need to update it in the future. Does that make sense? > > > > > > Well, no. You're adding (or changing, if you like) a userspace API. > > > We need to be precise about what that userspace API *means*, so that we > > > don't break it in the future when the implementation changes. You're > > > still being fuzzy above. > > > > > > I have no intention of adding an API like the ones you suggest above to > > > allow the user to specify what size pages to use. That seems very strange > > > to me; how should the user (or sysadmin, or application) know what size is > > > best for the kernel to use to cache files? Instead, the kernel observes > > > the usage pattern of the file (through the readahead mechanism) and grows > > > the allocation size to fit what the kernel thinks will be most effective. > > > > > > I do honour some of the existing hints that userspace can provide; eg > > > VM_HUGEPAGE makes the pagefault path allocate PMD sized pages (if it can). > > > > Right, so since VM_HUGEPAGE makes the kernel allocate PMD mapped THP > > if it can, then I want to know if the page is actually a PMD mapped > > THP or not. The implementation and documentation that I'm adding seem > > consistent with that AFAICT, but sorry if I missed something. > > So what userspace cares about is that the kernel is mapping the > memory with a PMD entry; it doesn't care whether the file is > being cached in 2MB (or larger) chunks. So we can drop the 'THP' > from all of this, and just call the bit the PMD mapping bit? I've thought about this a bit, but I have a couple of problems: 1. It's a bit difficult to implement this for hugetlb pages, or at least I haven't found a reasonably simple way to implement this for hugetlb pages. hugetlb ranges are handled by pagemap_hugetlb_range(ptep, hmask, ...). I can't find a way to uncover whether ptep points to a pmd_t or pud_t or even pte_t with contig PTE bit set. I can also easily surmise the size of the page from the hmask, but I need to know what's the native page size and what arch I'm running on to convert a page size to "is PMD mapped or not'' information. Very sorry if I missed an easy way to do this. 2. Semantically I'm not sure it makes sense to tell the user if a page is PMD hugetlb or not. For THP I think it makes somewhat sense because the userspace asks for hugepages via MADV_HUGEPAGE or huge=always, and 'huge' roughly here means 'PMD mapped', per your statement that for VM_HUGEPAGE makes the kernel try to allocate PMD size pages. For hugetlb, the userspace never asks for 'huge' pages or PMD mappings per say, they ask for a specific size, and it's considered an implementation detail how the mapping is achieved, and may not even be backwards compatible.