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

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Message-ID: <ZszlQEqdDl4vt43M@tiehlicka>
Date: Mon, 26 Aug 2024 22:27:44 +0200
From: Michal Hocko <mhocko@...e.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
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
Subject: Re: [PATCH 1/2] bcachefs: do not use PF_MEMALLOC_NORECLAIM

On Mon 26-08-24 16:00:56, Kent Overstreet wrote:
> On Mon, Aug 26, 2024 at 09:58:08PM GMT, Michal Hocko wrote:
> > On Mon 26-08-24 15:39:47, Kent Overstreet wrote:
> > > On Mon, Aug 26, 2024 at 10:47:12AM GMT, 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.
> > > 
> > > yeah, eesh, nack.
> > 
> > Sure, you can NAK this but then deal with the lack of the PF flag by
> > other means. We have made it clear that PF_MEMALLOC_NORECLAIM is not we
> > are going to support at the MM level. 
> > 
> > I have done your homework and shown that it is really easy
> > to use gfp flags directly. The net result is passing gfp flag down to
> > two functions. Sure part of it is ugglier by having several different
> > callbacks implementing it but still manageable. Without too much churn.
> > 
> > So do whatever you like in the bcache code but do not rely on something
> > that is unsupported by the MM layer which you have sneaked in without an
> > agreement.
> 
> Michal, you're being damned hostile, while posting code you haven't even
> tried to compile. Seriously, dude?
> 
> How about sticking to the technical issues at hand instead of saying
> "this is mm, so my way or the highway?". We're all kernel developers
> here, this is not what we do.

Kent, we do respect review feedback. You are clearly fine ignoring it
when you feels like it (eab0af905bfc ("mm: introduce
PF_MEMALLOC_NORECLAIM, PF_MEMALLOC_NOWARN") is a clear example of it).

I have already made my arguments (repeatedly) why implicit nowait
allocation context is tricky and problematic. Your response is that you
simply "do no buy it" which is a highly technical argument.

-- 
Michal Hocko
SUSE Labs