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

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Message-ID: <v664cj6evwv7zu3b77gf2lx6dv5sp4qp2rm7jjysddi2wc2uzl@qvnj4kmc6xhq>
Date: Mon, 2 Sep 2024 04:52:49 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Michal Hocko <mhocko@...e.com>
Cc: Dave Chinner <david@...morbit.com>, 
	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
Subject: Re: [PATCH 1/2 v2] bcachefs: do not use PF_MEMALLOC_NORECLAIM

On Mon, Sep 02, 2024 at 10:41:31AM GMT, Michal Hocko wrote:
> On Sun 01-09-24 21:35:30, Kent Overstreet wrote:
> [...]
> > But I am saying that kmalloc(__GFP_NOFAIL) _should_ fail and return NULL
> > in the case of bugs, because that's going to be an improvement w.r.t.
> > system robustness, in exactly the same way we don't use BUG_ON() if it's
> > something that we can't guarantee won't happen in the wild - we WARN()
> > and try to handle the error as best we can.
> 
> We have discussed that in a different email thread. And I have to say
> that I am not convinced that returning NULL makes a broken code much
> better. Why? Because we can expect that broken NOFAIL users will not have a
> error checking path. Even valid NOFAIL users will not have one because
> they _know_ they do not have a different than retry for ever recovery
> path. 

You mean where I asked you for a link to the discussion and rationale
you claimed had happened? Still waiting on that

> That means that an unexpected NULL return either means OOPS or a subtle
> silent error - e.g. memory corruption. The former is a actually a saner
> recovery model because the execution is stopped before more harm can be
> done. I suspect most of those buggy users will simply OOPS but
> systematically checking for latter is a lot of work and needs to be
> constantly because code evolves...
> 
> I have tried to argue that if allocator cannot or refuse to satisfy
> GFP_NOFAIL request because it is trying to use unsupported allocation
> mode or size then we should terminate the allocation context. That would
> make the API more predictable and therefore safer to use.

You're arguing that we treat it like an oops?

Yeah, enough of this insanity.