From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Date: Mon, 18 Oct 2004 19:14:12 +0000 Subject: Re: [PATCH] General purpose zeroed page slab Message-Id: <20041018191412.GJ16153@parcelfarce.linux.theplanet.co.uk> List-Id: References: In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Luck, Tony" Cc: Matthew Wilcox , "Martin K. Petersen" , Andi Kleen , linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, akpm@osdl.org On Mon, Oct 18, 2004 at 12:03:04PM -0700, Luck, Tony wrote: > >It's probably worth doing this with a static cachep in slab.c and only > >exposing a get_zeroed_page() / free_zeroed_page() interface, with the > >latter doing the memset to 0. I disagree with Andi over the dumbness > >of zeroing the whole page. That makes it cache-hot, which is what you > >want from a page you allocate from slab. > > We started this discussion with the plan of using this interface to > allocate/free page tables at all levels in the page table hierarchy > (rather than maintain a special purpose "quicklist" allocator for each > level). This is a somewhat specialized usage in that we know that we > have a completely zeroed page when we free ... so we really don't > want the overhead of zeroing it again. Ah, I'm not a VM weenie, so I didn't know this was guaranteed ;-) > There is also somewhat limited > benefit to the cache hotness argument here as most page tables (especially > higher-order ones) are used very sparsely. > > That said, the idea to expose this slab only through a specific API > should calm fears about accidental mis-use (with people freeing a page > that isn't all zeroes). So alloc_zeroed_page(), free_zeroed_page(), zero_and_free_page() interfaces with a CONFIG option to check that the page passed to free_zeroed_page() is already zero? -- "Next the statesmen will invent cheap lies, putting the blame upon the nation that is attacked, and every man will be glad of those conscience-soothing falsities, and will diligently study them, and refuse to examine any refutations of them; and thus he will by and by convince himself that the war is just, and will thank God for the better sleep he enjoys after this process of grotesque self-deception." -- Mark Twain