From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jes Sorensen Date: Tue, 22 Feb 2005 14:41:04 +0000 Subject: Re: [patch -mm series] ia64 specific /dev/mem handlers Message-Id: List-Id: References: <16923.193.128608.607599@jaguar.mkp.net> <20050222020309.4289504c.akpm@osdl.org> In-Reply-To: <20050222020309.4289504c.akpm@osdl.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Andrew Morton Cc: linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org >>>>> "Andrew" = Andrew Morton writes: Andrew> jes@trained-monkey.org (Jes Sorensen) wrote: >> Hi, >> >> This patch introduces ia64 specific read/write handlers for >> /dev/mem access which is needed to avoid uncached pages to be >> accessed through the cached kernel window which can lead to random >> corruption. It also introduces a new page-flag PG_uncached which >> will be used to mark the uncached pages. I assume this may be >> useful to other architectures as well where the CPU may use >> speculative reads which conflict with uncached access. In addition >> I moved do_write_mem to be under ARCH_HAS_DEV_MEM as it's only ever >> used if that is defined. Andrew> Is it possible to avoid consuming a page flag? Andrew> If this is an ia64-only (or 64-bit-only) thing I guess we Andrew> could use bit 32. Hi Andrew, I don't think we can get away with it, I think most modern architectures have this problem. I've been trying to avoid it for several iterations but I kept getting pushed towards this. >> + if (page->flags & PG_uncached) Andrew> dude. That ain't gonna work ;) Pardon my lack of clue, but why not? Maybe I should explain how I use it - in the mspec driver I pull a chunk of pages out using alloc_pages and convert them all to uncached (I need to do a chunk due to the speculation restrictions on ia64) and then stick them into a special allocator and set page->PG_uncached when doing so (the allocator is not tied to the uncached so I posted the patch to Tony Luck). Ie. once the pages have been grabbed with alloc_pages() and converted to uncached, they are never put back into the generic pool. It may be there are other places which needs to learn to respect the flag? Cheers, Jes