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Tue, 10 Apr 2018 11:54:49 -0400 Date: Tue, 10 Apr 2018 08:54:42 -0700 From: Matthew Wilcox To: Christopher Lameter Cc: linux-mm@kvack.org, Matthew Wilcox , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , linux-kernel@vger.kernel.org, Jan Kara , Jeff Layton , Mel Gorman , stable@vger.kernel.org Subject: Re: [PATCH 1/2] slab: __GFP_ZERO is incompatible with a constructor Message-ID: <20180410155442.GA3614@bombadil.infradead.org> References: <20180410125351.15837-1-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.2 (2017-12-15) Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Tue, Apr 10, 2018 at 09:21:20AM -0500, Christopher Lameter wrote: > On Tue, 10 Apr 2018, Matthew Wilcox wrote: > > > __GFP_ZERO requests that the object be initialised to all-zeroes, > > while the purpose of a constructor is to initialise an object to a > > particular pattern. We cannot do both. Add a warning to catch any > > users who mistakenly pass a __GFP_ZERO flag when allocating a slab with > > a constructor. > > Can we move this check out of the critical paths and check for > a ctor and GFP_ZERO when calling the page allocator? F.e. in > allocate_slab()? Are you willing to have this kind of bug go uncaught for a while? In this specific case, __GFP_ZERO was only being passed on a few of the calls to kmem_cache_alloc. So we'd happily trash the constructed object any time we didn't allocate a page. I appreciate it's a tradeoff, and we don't want to clutter the critical path unnecessarily.