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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pnRV2-0097aB-37; Fri, 14 Apr 2023 22:02:32 +0000 Date: Fri, 14 Apr 2023 23:02:32 +0100 From: Matthew Wilcox To: Bernd Schubert Cc: "linux-fsdevel@vger.kernel.org" , Miklos Szeredi , Andrew Morton , linux-mm@kvack.org Subject: Re: sequential 1MB mmap read ends in 1 page sync read-ahead Message-ID: References: <1e88b8ed-5f17-c42e-9646-6a97efd9f99c@fastmail.fm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: BE26E1C0021 X-Stat-Signature: whptwap6j6dmn8igj7uhgst3zwfgonis X-Rspam-User: X-HE-Tag: 1681509759-652133 X-HE-Meta: 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 nm/tl1oF sWuuW+9KPQx/02IWU6NWkUkOQuOfwU/T2TXiepjJOCNuhKaabZZxsnExMmbG8UL7dr9v1j/WmPPVGsMRIpKixU2bq6d2LcOWHu9oL7nt2XvOgDehoLYgTxKAOsRh49gFXRC5IABkhwdwhnnBYwzInZa8OXvJtuoQFR84DTAtzP2Ujc+7UvLhvnkFL29cMnFjhnMCsTELW6auBgl0cFmj0dyDd/Q== 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 Fri, Apr 14, 2023 at 10:47:39PM +0200, Bernd Schubert wrote: > > Up to bs=512K it works fine, 1M (and for what it matters > > already 768K) introduce the order=0 issue. > > Hmm, I replaced memcpy with dumb version, that copies byte by byte - problem > gone. Is it possible that the optimized memcpy causes caused kind of random > memory access and confuses mm / readahead? > And somehow your memcpy or system is not doing that? Oh, that would make sense! If the memcpy() works high-to-low, then you'd see exactly the behaviour you're reporting. Whereas low-to-high results in the behaviour I'm seeing. Hm. I don't know what to do about that. Maybe a "sufficiently large" memcpy should call posix_madvise(src, n, POSIX_MADV_WILLNEED)