linux-kernel - Re: [PATCH 2/2] page cache: Mask off unwanted GFP flags

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Message-ID: <20180410140223.GE22118@bombadil.infradead.org>
Date:   Tue, 10 Apr 2018 07:02:23 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     linux-mm@...ck.org, Matthew Wilcox <mawilcox@...rosoft.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        stable@...r.kernel.org, jaegeuk@...nel.org
Subject: Re: [PATCH 2/2] page cache: Mask off unwanted GFP flags

On Tue, Apr 10, 2018 at 10:45:45PM +0900, Minchan Kim wrote:
> On Tue, Apr 10, 2018 at 05:53:51AM -0700, Matthew Wilcox wrote:
> > From: Matthew Wilcox <mawilcox@...rosoft.com>
> > 
> > The page cache has used the mapping's GFP flags for allocating
> > radix tree nodes for a long time.  It took care to always mask off the
> > __GFP_HIGHMEM flag, and masked off other flags in other paths, but the
> > __GFP_ZERO flag was still able to sneak through.  The __GFP_DMA and
> > __GFP_DMA32 flags would also have been able to sneak through if they
> > were ever used.  Fix them all by using GFP_RECLAIM_MASK at the innermost
> > location, and remove it from earlier in the callchain.
> > 
> > Fixes: 19f99cee206c ("f2fs: add core inode operations")
> 
> Why this patch fix 19f99cee206c instead of 449dd6984d0e?
> F2FS doesn't have any problem before introducing 449dd6984d0e?

Well, there's the problem.  This bug is the combination of three different
things:

1. The working set code relying on list_empty.
2. The page cache not filtering out the bad flags.
3. F2FS specifying a flag nobody had ever specified before.

So what single patch does this patch fix?  I don't think it really matters.