linux-kernel - Re: [PATCH 2/2] mm: drop PF_MEMALLOC

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Message-ID: <dertpwh33crhjpghdsn5n3svrwhrv7cv446hyrqdblc3wcw3pp@a7ds4wsgvehg>
Date: Mon, 26 Aug 2024 15:04:19 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Michal Hocko <mhocko@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Hellwig <hch@....de>, Yafang Shao <laoar.shao@...il.com>, 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 2/2] mm: drop PF_MEMALLOC_NORECLAIM

On Mon, Aug 26, 2024 at 10:47:13AM GMT, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> There is no existing user of the flag and the flag is dangerous because
> a nested allocation context can use GFP_NOFAIL which could cause
> unexpected failure. Such a code would be hard to maintain because it
> could be deeper in the call chain.
> 
> PF_MEMALLOC_NORECLAIM has been added even when it was pointed out [1]
> that such a allocation contex is inherently unsafe if the context
> doesn't fully control all allocations called from this context.

I don't really buy the unsafety argument; if it applies to anything, it
applies to GFP_NOFAIL - but we recently grew warnings about unsafe uses
for it, so I don't see it as a great concern.

GFP_NORECLAIM is frequently desirable as a hint about the latency
requirements of a codepath; "don't try too hard, I've got fallbacks and
I'm in a codepath where I don't want to block too long".

I expect PF_MEMALLOC_NORECLAIM will find legitimate uses.