From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Stephen C. Tweedie" MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <14186.31507.833263.846717@dukat.scot.redhat.com> Date: Fri, 18 Jun 1999 18:00:03 +0100 (BST) Subject: Re: filecache/swapcache questions In-Reply-To: <199906180020.RAA02498@google.engr.sgi.com> References: <14185.34250.163041.796165@dukat.scot.redhat.com> <199906180020.RAA02498@google.engr.sgi.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Kanoj Sarcar Cc: "Stephen C. Tweedie" , riel@nl.linux.org, linux-mm@kvack.org List-ID: Hi, On Thu, 17 Jun 1999 17:20:10 -0700 (PDT), kanoj@google.engr.sgi.com (Kanoj Sarcar) said: > Interesting scenario ... unfortunately, I am getting confused. > I am trying to lay out the steps in your example here: > Step 1: P1 and P2 sharing a page which is not in core, is out on > swap at swap handle X, swap_count(X) = 2 (P1 + P2) > Step 2: P1 writes to page. > Step 2a: swap_in reads in the page into core into page A, > page_count(A) = 2 (swapcache + P1), A.offset = X, > swap_count (X)= 2 (P2 + swapcache) Yes. Exactly. > So, what am I missing, since your example does not end up with > page_count = 1 and swap_count(page offset/swaphandle) = 1? > I did give an alternative scenario involving an exitting process, > do you believe that one? Yes --- I'd missed the fact that you wanted swap_count to be one as well as page count. > While I have your attention, I think I found a bug in the > sys_swapoff algorithm ... basically, it needs to also look > at swap_lockmap. Say an exitting process fired off some async > swap ins just before it exitted, and a bunch of these are in > flight (swap_lockmaps are set, as are swap_map, from swapcache). > The swap device gets deleted (with a printk warning message due > to non zero swap_map count). Finally, the old async swap in's > start terminating, invoking swap_after_unlock_page. Interesting > things could happen, depending on whether the swap id has been > reallocated or not ... Is there any protection against this > scenario? Yes --- try_to_unuse calls read_swap_cache() with wait==1, so we always wait for the IO to complete before swapoff can complete. At least, that's the theory. :) --Stephen -- To unsubscribe, send a message with 'unsubscribe linux-mm my@address' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://humbolt.geo.uu.nl/Linux-MM/