linux-kernel - Re: [PATCH 1/2 v2] bcachefs: do not use PF_MEMALLOC

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Message-ID: <Zs6jFb953AR2Raec@dread.disaster.area>
Date: Wed, 28 Aug 2024 14:09:57 +1000
From: Dave Chinner <david@...morbit.com>
To: Michal Hocko <mhocko@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>, Yafang Shao <laoar.shao@...il.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>, jack@...e.cz,
	Christian Brauner <brauner@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-bcachefs@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 1/2 v2] bcachefs: do not use PF_MEMALLOC_NORECLAIM

On Tue, Aug 27, 2024 at 08:15:43AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> bch2_new_inode relies on PF_MEMALLOC_NORECLAIM to try to allocate a new
> inode to achieve GFP_NOWAIT semantic while holding locks. If this
> allocation fails it will drop locks and use GFP_NOFS allocation context.
> 
> We would like to drop PF_MEMALLOC_NORECLAIM because it is really
> dangerous to use if the caller doesn't control the full call chain with
> this flag set. E.g. if any of the function down the chain needed
> GFP_NOFAIL request the PF_MEMALLOC_NORECLAIM would override this and
> cause unexpected failure.
> 
> While this is not the case in this particular case using the scoped gfp
> semantic is not really needed bacause we can easily pus the allocation
> context down the chain without too much clutter.
> 
> Acked-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Looks good to me.

Reviewed-by: Dave Chinner <dchinner@...hat.com>

-- 
Dave Chinner
david@...morbit.com