From mboxrd@z Thu Jan 1 00:00:00 1970 From: kanoj@google.engr.sgi.com (Kanoj Sarcar) Message-Id: <199906180020.RAA02498@google.engr.sgi.com> Subject: Re: filecache/swapcache questions Date: Thu, 17 Jun 1999 17:20:10 -0700 (PDT) In-Reply-To: <14185.34250.163041.796165@dukat.scot.redhat.com> from "Stephen C. Tweedie" at Jun 18, 99 00:33:30 am MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: "Stephen C. Tweedie" Cc: riel@nl.linux.org, linux-mm@kvack.org List-ID: > > Hi, > > On Tue, 15 Jun 1999 09:32:19 +0200 (CEST), Rik van Riel > said: > > >> How will it be possible for a page to be in the swapcache, for its > >> reference count to be 1 (which has been checked just before), and for > >> its swap_count(page->offset) to also be 1? I can see this being > >> possible only if an unmap/exit path might lazily leave a anonymous > >> page in the swap cache, but I don't believe that happens. > > > It does happen. We use a 'two-stage' reclamation process instead > > of page aging. It seems to work wonderfully -- nice page aging > > properties without the overhead. > > Much more than that: if we take a write fault to a page which is shared > on swap by two processes, then we bring it into cache and take a > copy-on-write, leaving one copy in the swap cache (reference one: it is > _only_ in use by the swap cache now), and the other copy being reference > by the faulting process. > > --Stephen > -- 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) Step 2b: P1 incurs do_wp_page on the page, gets a new page. The old page A ends up with a page_count = 1 (swapcache), and swap_count (X) stays at 2. 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? 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? Thanks. Kanoj kanoj@engr.sgi.com -- 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/