From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Date: Thu, 03 Mar 2005 06:11:53 +0000 Subject: Re: Page fault scalability patch V18: Drop first acquisition of ptl Message-Id: <1109830313.5680.183.camel@gaston> List-Id: References: <20050302174507.7991af94.akpm@osdl.org> <20050302185508.4cd2f618.akpm@osdl.org> <20050302201425.2b994195.akpm@osdl.org> <16934.39386.686708.768378@cargo.ozlabs.ibm.com> <20050302213831.7e6449eb.davem@davemloft.net> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Christoph Lameter Cc: "David S. Miller" , Paul Mackerras , Andrew Morton , Linux Kernel list , linux-ia64@vger.kernel.org, Anton Blanchard On Wed, 2005-03-02 at 21:51 -0800, Christoph Lameter wrote: > On Wed, 2 Mar 2005, David S. Miller wrote: > > > Actually, I guess I could do the pte_cmpxchg() stuff, but only if it's > > used to "add" access. If the TLB miss handler races, we just go into > > the handle_mm_fault() path unnecessarily in order to synchronize. > > > > However, if this pte_cmpxchg() thing is used for removing access, then > > sparc64 can't use it. In such a case a race in the TLB handler would > > result in using an invalid PTE. I could "spin" on some lock bit, but > > there is no way I'm adding instructions to the carefully constructed > > TLB miss handler assembler on sparc64 just for that :-) > > There is no need to provide pte_cmpxchg. If the arch does not support > cmpxchg on ptes (CONFIG_ATOMIC_TABLE_OPS not defined) > then it will fall back to using pte_get_and_clear while holding the > page_table_lock to insure that the entry is not touched while performing > the comparison. Nah, this is wrong :) We actually _want_ pte_cmpxchg on ppc64, because we can do the stuff, but it requires some careful manipulation of some bits in the PTE that are beyond linux common layer understanding :) Like the BUSY bit which is a lock bit for arbitrating with the hash fault handler for example. Also, if it's ever used to cmpxchg from anything but a !present PTE, it will need additional massaging (like the COW case where we just "replace" a PTE with set_pte). We also need to preserve some bits in there that indicate if the PTE was in the hash table and where in the hash so we can flush it afterward. Ben.