Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks

From: Yu Zhao <yuzhao@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Andi Kleen" <ak@linux.intel.com>,
	"Aneesh Kumar" <aneesh.kumar@linux.ibm.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Hillf Danton" <hdanton@sina.com>, "Jens Axboe" <axboe@kernel.dk>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Mel Gorman" <mgorman@suse.de>,
	"Michael Larabel" <Michael@michaellarabel.com>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Mike Rapoport" <rppt@kernel.org>, "Tejun Heo" <tj@kernel.org>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Will Deacon" <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
	page-reclaim@google.com, "Brian Geffon" <bgeffon@google.com>,
	"Jan Alexander Steffens" <heftig@archlinux.org>,
	"Oleksandr Natalenko" <oleksandr@natalenko.name>,
	"Steven Barrett" <steven@liquorix.net>,
	"Suleiman Souhlal" <suleiman@google.com>,
	"Daniel Byrne" <djbyrne@mtu.edu>,
	"Donald Carr" <d@chaos-reins.com>,
	"Holger Hoffstätte" <holger@applied-asynchrony.com>,
	"Konstantin Kharlamov" <Hi-Angel@yandex.ru>,
	"Shuang Zhai" <szhai2@cs.rochester.edu>,
	"Sofia Trinh" <sofia.trinh@edi.works>,
	"Vaibhav Jain" <vaibhav@linux.ibm.com>
Subject: Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks
Date: Tue, 18 Oct 2022 23:51:00 -0600	[thread overview]
Message-ID: <CAOUHufa9+FTO3Pv-5jC-e3S5goPsUGu-5KcPVHa4bWb0X+d2ug@mail.gmail.com> (raw)
In-Reply-To: <Y0go8wWtdcyH1+Ch@hirez.programming.kicks-ass.net>

On Thu, Oct 13, 2022 at 9:04 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Aug 15, 2022 at 01:13:27AM -0600, Yu Zhao wrote:
> > +     for (i = pmd_index(start), addr = start; addr != end; i++, addr = next) {
> > +             pmd_t val = pmd_read_atomic(pmd + i);
> > +
> > +             /* for pmd_read_atomic() */
> > +             barrier();
>
> Please clarify the above. This is an entirely inadequate ordering
> comment.

If it's acceptable, I'll copy what we have in
pmd_none_or_clear_bad_unless_trans_huge():

        pmd_t pmdval = pmd_read_atomic(pmd);

       /* See pmd_none_or_trans_huge_or_clear_bad for info on barrier */
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
        barrier();
#endif

        if (pmd_none(pmdval))
                return 1;

pmd_read_atomic() should have a built-in READ_ONCE() in the first
place. If we have to use pmd_read_atomic(), it means we are not under
PMD PTL. So we can also race with pte_alloc(), regardless of THP
split. In this case, compiler reordering probably won't cause any real
damage, but technically not having barrier() is still a bug and will
trigger KCSAN warnings, I think.