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

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Date:   Thu, 14 Sep 2023 11:31:22 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Jaeseon Sim <jason.sim@...sung.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> [230913 22:49]:
> > * 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?

Must users in the VMA space have complicated locking schemes which
require no sleeping during a store operation.  Most operations will drop
the lock and re-try with GFP_KERNEL when using the internal lock (see
mas_nomem()).

> I wonder if other callers for mas_node_count might have similar issue.

The external callers who need GFP_KERNEL are either using
mas_store_gfp() or mas_prealloc to set up a store prior to taking a
series of other locks.

During a mas_prealloc() or mas_expected_entries() call, we set the
MA_STATE_PREALLOC flag to indicate that there are nodes preallocated.
This is to catch users who call mas_node_count() and require increased
allocations when allocations should not be taken.  You can see this flag
directly below the line you modified.

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

Thanks.  Please test with lockdep but I don't see a nesting lock issue
with fork and this change.

> 
> 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