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Shutemov" , Andrew Morton , Catalin Marinas , Evgenii Stepanov , Jann Horn , Linux ARM , linux-mm@kvack.org, kernel test robot , Linux API , linux-doc@vger.kernel.org Subject: Re: [PATCH v4] mm: introduce reference pages Message-ID: References: <20210619092002.1791322-1-pcc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210619092002.1791322-1-pcc@google.com> Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=raSjpWt4; dmarc=none; spf=none (imf13.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org X-Rspamd-Server: rspam02 X-Stat-Signature: dj9f4iunio84sgrbj86tbwt5q1s631xr X-Rspamd-Queue-Id: D5169E000279 X-HE-Tag: 1624908917-948212 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 Sat, Jun 19, 2021 at 02:20:02AM -0700, Peter Collingbourne wrote: > +void prep_refpage_private_data(struct refpage_private_data *priv) > +{ > + u8 *addr = page_address(priv->refpage); > + u8 pattern = addr[0]; > + int i; > + > + for (i = 1; i != PAGE_SIZE; ++i) > + if (addr[i] != pattern) > + return; > + > + priv->optzn_kind = REFPAGE_OPTZN_PATTERN; > + priv->optzn_info = pattern; > +} > + > +void copy_refpage(struct page *page, unsigned long addr, > + struct vm_area_struct *vma) > +{ > + struct refpage_private_data *priv = vma->vm_private_data; > + > + if (priv->optzn_kind == REFPAGE_OPTZN_PATTERN) > + memset(page_address(page), priv->optzn_info, PAGE_SIZE); > + else > + copy_user_highpage(page, priv->refpage, addr, vma); > +} I wonder if single-byte captures enough of the useful possibilities. In the kernel we have memset32() and memset64() [1] so we could support a larger pattern than just an 8-bit byte. It all depends what userspace would find useful. [1] Along with memset_p(), memset_l() and memset16() that aren't terribly relevant to this use case. Although maybe memset_l() would be the right one to use since there probably aren't too many 32-bit apps that want a 64-bit pattern and memset64() might not be the fastest on a 32-bit kernel).