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Shutemov" To: Hugh Dickins Cc: Andrew Morton , "Kirill A. Shutemov" , Yang Shi , Wang Yugui , Matthew Wilcox , Alistair Popple , Ralph Campbell , Zi Yan , Peter Xu , Will Deacon , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 03/11] mm: page_vma_mapped_walk(): use pmd_read_atomic() Message-ID: <20210610090617.e6qutzzj3jxcseyi@box.shutemov.name> References: <589b358c-febc-c88e-d4c2-7834b37fa7bf@google.com> <594c1f0-d396-5346-1f36-606872cddb18@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <594c1f0-d396-5346-1f36-606872cddb18@google.com> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 725E237E Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=shutemov-name.20150623.gappssmtp.com header.s=20150623 header.b=YFsPavJK; spf=none (imf04.hostedemail.com: domain of kirill@shutemov.name has no SPF policy when checking 209.85.208.178) smtp.mailfrom=kirill@shutemov.name; dmarc=none X-Stat-Signature: 6xj1mpkeqe1ubif5ypumcryftzxg3ond X-HE-Tag: 1623315960-788314 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 Wed, Jun 09, 2021 at 11:38:11PM -0700, Hugh Dickins wrote: > page_vma_mapped_walk() cleanup: use pmd_read_atomic() with barrier() > instead of READ_ONCE() for pmde: some architectures (e.g. i386 with PAE) > have a multi-word pmd entry, for which READ_ONCE() is not good enough. > > Signed-off-by: Hugh Dickins > Cc: > --- > mm/page_vma_mapped.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index 7c0504641fb8..973c3c4e72cc 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -182,13 +182,16 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > pud = pud_offset(p4d, pvmw->address); > if (!pud_present(*pud)) > return false; > + > pvmw->pmd = pmd_offset(pud, pvmw->address); > /* > * Make sure the pmd value isn't cached in a register by the > * compiler and used as a stale value after we've observed a > * subsequent update. > */ > - pmde = READ_ONCE(*pvmw->pmd); > + pmde = pmd_read_atomic(pvmw->pmd); > + barrier(); > + Hm. It makes me wounder if barrier() has to be part of pmd_read_atomic(). mm/hmm.c uses the same pattern as you are and I tend to think that the rest of pmd_read_atomic() users may be broken. Am I wrong? -- Kirill A. Shutemov