From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Thu, 15 Feb 2001 12:27:42 -0500 (EST) From: Ben LaHaise Subject: Re: x86 ptep_get_and_clear question In-Reply-To: <200102151723.JAA43255@google.engr.sgi.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Kanoj Sarcar Cc: Jamie Lokier , linux-mm@kvack.org, mingo@redhat.com, alan@redhat.com List-ID: On Thu, 15 Feb 2001, Kanoj Sarcar wrote: > continue with my previous example, instead of pulling new examples. > > Look in mm/mprotect.c. Look at the call sequence change_protection() -> ... > change_pte_range(). Specifically at the sequence: > > entry = ptep_get_and_clear(pte); > set_pte(pte, pte_modify(entry, newprot)); > > Go ahead and pull your x86 specs, and prove to me that between the > ptep_get_and_clear(), which zeroes out the pte (specifically, when the > dirty bit is not set), processor 2 can not come in and set the dirty > bit on the in-memory pte. Which immediately gets overwritten by the > set_pte(). For an example of how this can happen, look at my previous > postings. Look at the specs. The processor uses read-modify-write cycles to update the accessed and dirty bits. If the in memory pte is either not present or writable, the processor will take a page fault. > Jamie's example misses the point in the sense that at the very beginning, > when he says "Processor 2 has recently done some writes", processor 2 has > made sure that the dirty bit is set in the in-memory pte. So, although > processor 1 clears the entire pte, the set_pte() will set the dirty bit, > and no information is lost. Even if processor 2 tries writing between > the ptep_get_and_clear() and set_pte(). Whether Jamie was trying to > illustrate a different problem, I am not sure. All I am trying to say > is that the "dirty bit lost on smp x86" still exists, ptep_get_and_clear > does not do anything to fix it. Yes it does. Write a test program like I did. The processor does take a page fault. -ben -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux.eu.org/Linux-MM/