linux-kernel - Re: [PATCH] maple_tree: use GFP_KERNEL on mas_node

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Message-ID: <20230914024937epcms1p24be0b59caeefd7966bec81ac11feff8f@epcms1p2>
Date:   Thu, 14 Sep 2023 11:49:37 +0900
From:   Jaeseon Sim <jason.sim@...sung.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>
CC:     Matthew Wilcox <willy@...radead.org>,
        Peng Zhang <zhangpeng.00@...edance.com>,
        "surenb@...gle.com" <surenb@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jaewon31.kim@...il.com" <jaewon31.kim@...il.com>,
        "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>
Subject: Re: [PATCH] maple_tree: use GFP_KERNEL on mas_node_count

> * Jaeseon Sim <jason.sim@...sung.com> [230907 00:41]:
> > > On Thu, Sep 07, 2023 at 12:02:02PM +0800, Peng Zhang wrote:
> > > > 
> > > > 
> > > > 在 2023/9/7 11:49, Matthew Wilcox 写道:
> > > > > On Thu, Sep 07, 2023 at 12:39:14PM +0900, 심재선 wrote:
> > > > > > Use GFP_KERNEL on mas_node_count instead of GFP_NOWAIT | __GFP_NOWARN
> > > > > > in order to allow memory reclaim.
> > > > There are many paths that call maple tree's mas_node_count(). Some paths
> > > > cannot reclaim memory.
> > > 
> > > Right ... but we should be handling the ENOMEM inside the maple tree and
> > > allocating some nodes with GFP_KERNEL instead of failing fork().
> > > 
> > > > > What testing did you do of this patch?  In particular, did you try it
> > > > > with lockdep enabled?
> > I did power on/off test with this patch.
> > I did not try it with lockdep enabled.
> 
> To accomplish the same result, but with a much smaller scope that will
> work with lockdep, I would suggest changing mas_expected_entries() to
> use mas_node_count_gfp() (which already exists) and pass in GFP_KERNEL.
> 
> Since fork is the only current user of mas_expected_entries(), this
> won't break other users and we can deal with changing it for others if
> it is needed.
> 
> If we do go this route, please add a note in the documentation about
> using GFP_KERNEL.
> 
> Willy, does that work for you?
> 
> Thanks,
> Liam

Dear Liam,
Can I ask you the reason why mas_node_count is using GFP_NOWAIT | __GFP_NOWARN?
I wonder if other callers for mas_node_count might have similar issue.

From your opinion, I'll post v2 patch as follows

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index ee1ff0c59fd7..b0229271c24e 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5574,7 +5574,7 @@ int mas_expected_entries(struct ma_state *mas, unsigned long nr_entries)
 	/* Internal nodes */
 	nr_nodes += DIV_ROUND_UP(nr_nodes, nonleaf_cap);
 	/* Add working room for split (2 nodes) + new parents */
-	mas_node_count(mas, nr_nodes + 3);
+	mas_node_count_gfp(mas, nr_nodes + 3, GFP_KERNEL);
 
 	/* Detect if allocations run out */
 	mas->mas_flags |= MA_STATE_PREALLOC;
-- 
2.17.1

Thanks
Jaeseon