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

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Message-ID: <ZszAI7oYsh7FvGgg@casper.infradead.org>
Date: Mon, 26 Aug 2024 18:49:23 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Michal Hocko <mhocko@...e.com>
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
Subject: Re: [PATCH 2/2] mm: drop PF_MEMALLOC_NORECLAIM

On Mon, Aug 26, 2024 at 06:51:55PM +0200, Michal Hocko wrote:
> On Mon 26-08-24 14:59:29, Matthew Wilcox wrote:
> > On Mon, Aug 26, 2024 at 10:47:13AM +0200, 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.
> > 
> > Wouldn't a straight-up revert of eab0af905bfc be cleaner?  Or is there
> > a reason to keep PF_MEMALLOC_NOWARN?
> 
> I wanted to make it PF_MEMALLOC_NORECLAIM specific. I do not have a
> strong case against PF_MEMALLOC_NOWARN TBH. It is a hack because the
> scope is claiming something about all allocations within the scope
> without necessarily knowing all of them (including potential future
> changes). But NOWARN is not really harmful so I do not care strongly.
> 
> If a plan revert is preferably, I will go with it.

There aren't any other users of PF_MEMALLOC_NOWARN and it definitely
seems like something you want at a callsite rather than blanket for every
allocation below this point.  We don't seem to have many PF_ flags left,
so let's not keep it around if there's no immediate plans for it.