From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 07F826B0275 for ; Tue, 15 May 2018 07:54:11 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id x21-v6so12957261pfn.23 for ; Tue, 15 May 2018 04:54:10 -0700 (PDT) Received: from bombadil.infradead.org (bombadil.infradead.org. [2607:7c80:54:e::133]) by mx.google.com with ESMTPS id o1-v6si10644592plk.577.2018.05.15.04.54.09 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 15 May 2018 04:54:09 -0700 (PDT) Date: Tue, 15 May 2018 04:54:04 -0700 From: Matthew Wilcox Subject: Re: [External] Re: [PATCH 2/3] include/linux/gfp.h: use unsigned int in gfp_zone Message-ID: <20180515115404.GD31599@bombadil.infradead.org> References: <20180504154004.GB29829@bombadil.infradead.org> <20180506134814.GB7362@bombadil.infradead.org> <20180506185532.GA13604@bombadil.infradead.org> <20180507184410.GA12361@bombadil.infradead.org> <20180507212500.bdphwfhk55w6vlbb@twin.jikos.cz> <20180508002547.GA16338@bombadil.infradead.org> <20180509093659.jalprmufpwspya26@twin.jikos.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180509093659.jalprmufpwspya26@twin.jikos.cz> Sender: owner-linux-mm@kvack.org List-ID: To: dsterba@suse.cz, Huaisheng HS1 Ye , Michal Hocko , "akpm@linux-foundation.org" , "linux-mm@kvack.org" , "vbabka@suse.cz" , "mgorman@techsingularity.net" , "pasha.tatashin@oracle.com" , "alexander.levin@verizon.com" , "hannes@cmpxchg.org" , "penguin-kernel@I-love.SAKURA.ne.jp" , "colyli@suse.de" , NingTing Cheng , "linux-kernel@vger.kernel.org" On Wed, May 09, 2018 at 11:36:59AM +0200, David Sterba wrote: > On Mon, May 07, 2018 at 05:25:47PM -0700, Matthew Wilcox wrote: > > On Mon, May 07, 2018 at 11:25:01PM +0200, David Sterba wrote: > > > On Mon, May 07, 2018 at 11:44:10AM -0700, Matthew Wilcox wrote: > > > > But something like btrfs should almost certainly be using ~GFP_ZONEMASK. > > > > > > Agreed, the direct use of __GFP_DMA32 was added in 3ba7ab220e8918176c6f > > > to substitute GFP_NOFS, so the allocation flags are less restrictive but > > > still acceptable for allocation from slab. > > > > > > The requirement from btrfs is to avoid highmem, the 'must be acceptable > > > for slab' requirement is more MM internal and should have been hidden > > > under some opaque flag mask. There was no strong need for that at the > > > time. > > > > The GFP flags encode a multiple of different requirements. There's > > "What can the allocator do to free memory" and "what area of memory > > can the allocation come from". btrfs doesn't actually want to > > allocate memory from ZONE_MOVABLE or ZONE_DMA either. It's probably never > > been called with those particular flags set, but in the spirit of > > future-proofing btrfs, perhaps a patch like this is in order? > > > > ---- >8 ---- > > > > Subject: btrfs: Allocate extents from ZONE_NORMAL > > From: Matthew Wilcox > > > > If anyone ever passes a GFP_DMA or GFP_MOVABLE allocation flag to > > allocate_extent_state, it will try to allocate memory from the wrong zone. > > We just want to allocate memory from ZONE_NORMAL, so use GFP_RECLAIM_MASK > > to get what we want. > > Looks good to me. > > > Signed-off-by: Matthew Wilcox > > > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c > > index e99b329002cf..4e4a67b7b29d 100644 > > --- a/fs/btrfs/extent_io.c > > +++ b/fs/btrfs/extent_io.c > > @@ -216,12 +216,7 @@ static struct extent_state *alloc_extent_state(gfp_t mask) > > { > > struct extent_state *state; > > > > - /* > > - * The given mask might be not appropriate for the slab allocator, > > - * drop the unsupported bits > > - */ > > - mask &= ~(__GFP_DMA32|__GFP_HIGHMEM); > > I've noticed there's GFP_SLAB_BUG_MASK that's basically open coded here, > but this would not filter out the placement flags. > > > - state = kmem_cache_alloc(extent_state_cache, mask); > > I'd prefer some comment here, it's not obvious why the mask is used. Sorry, I dropped the ball on this. Would you prefer: /* Allocate from ZONE_NORMAL */ state = kmem_cache_alloc(extent_state_cache, mask & GFP_RECLAIM_MASK); or /* * Callers may pass in a mask which indicates they want to allocate * from a special zone, so clear those bits here rather than forcing * each caller to do it. We only want to use their mask to indicate * what strategies the memory allocator can use to free memory. */ state = kmem_cache_alloc(extent_state_cache, mask & GFP_RECLAIM_MASK); I tend to lean towards being more terse, but it's not about me, it's about whoever reads this code next. > > + state = kmem_cache_alloc(extent_state_cache, mask & GFP_RECLAIM_MASK); > > if (!state) > > return state; > > state->state = 0; >