From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <446C1E25.4080408@yahoo.com.au> Date: Thu, 18 May 2006 17:11:33 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [PATCH] mm: limit lowmem_reserve References: <200604021401.13331.kernel@kolivas.org> <200604081101.06066.kernel@kolivas.org> <443710F7.3040201@yahoo.com.au> <200605180011.43216.kernel@kolivas.org> In-Reply-To: <200605180011.43216.kernel@kolivas.org> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Con Kolivas Cc: Andrew Morton , ck@vds.kolivas.org, linux list , linux-mm@kvack.org List-ID: Con Kolivas wrote: > I hate to resuscitate this old thread, sorry but I'm still not sure we > resolved it and I want to make sure this issue isn't here as I see it. > OK, reclaim is slightly different. > On Saturday 08 April 2006 11:25, Nick Piggin wrote: > >>Con Kolivas wrote: >> >>>Ok. I think I presented enough information for why I thought >>>zone_watermark_ok would fail (for ZONE_DMA). With 16MB ZONE_DMA and a >>>vmsplit of 3GB we have a lowmem_reserve of 12MB. It's pretty hard to keep >>>that much ZONE_DMA free, I don't think I've ever seen that much free on >>>my ZONE_DMA on an ordinary desktop without any particular ZONE_DMA users. >>>Changing the tunable can make the lowmem_reserve larger than ZONE_DMA is >>>on any vmsplit too as far as I understand the ratio. >> >>Umm, for ZONE_DMA allocations, ZONE_DMA isn't a lower zone. So that >>12MB protection should never come into it (unless it is buggy?). > > > An i386 pc with a 3GB split will have approx > > 4000 pages ZONE_DMA > > and lowmem reserve will set lowmem reserve to approx > > 0 0 3000 3000 > > So if we call zone_watermark_ok with zone of ZONE_DMA and a classzone_idx of a > ZONE_NORMAL we will fail a zone_watermark_ok test almost always since it's > almost impossible to have 3000 free ZONE_DMA pages. I believe it can happen > like this: > > In balance_pgdat (vmscan.c:1116) if we end up with end_zone being a > ZONE_NORMAL zone, then during the scan below we (vmscan.c:1137) iterate over > all zones from 0 to end_zone and (vmscan.c:1147) we end up calling > > if (!zone_watermark_ok(zone, order, zone->pages_high, end_zone, 0)) > > which would now call zone_watermark_ok with zone being a ZONE_DMA, and > end_zone being the idx of a ZONE_NORMAL. > > So in summary if I'm not mistaken (and I'm good at being mistaken), if we > balance pgdat and find that ZONE_NORMAL or higher needs scanning, we'll end > up trying to flush the crap out of ZONE_DMA. If we're under memory pressure, kswapd will try to free up any candidate zone, yes. > > On my test case this indeed happens and my ZONE_DMA never goes below 3000 > pages free. If I lower the reserve even further my pages free gets stuck at > 3208 and can't free any more, and doesn't ever drop below that either. > > Here is the patch I was proposing What problem does that fix though? -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com -- 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-mm.org/ . Don't email: email@kvack.org