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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <y7vve7rbvpf7fq5puzszn5fwogm63dum4n47o36u5z5rn4fxxi@wspvw6mhwndq>
Date: Tue, 27 Aug 2024 03:05:29 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Michal Hocko <mhocko@...e.com>
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 Tue, Aug 27, 2024 at 08:58:39AM GMT, Michal Hocko wrote:
> On Tue 27-08-24 02:40:16, Kent Overstreet wrote:
> > On Tue, Aug 27, 2024 at 08:01:32AM GMT, Michal Hocko wrote:
> > > You are not really answering the main concern I have brought up though.
> > > I.e. GFP_NOFAIL being fundamentally incompatible with NORECLAIM semantic
> > > because the page allocator doesn't and will not support this allocation
> > > mode.  Scoped noreclaim semantic makes such a use much less visible
> > > because it can be deep in the scoped context there more error prone to
> > > introduce thus making the code harder to maintain. 
> > 
> > You're too attached to GFP_NOFAIL.
> 
> Unfortunatelly GFP_NOFAIL is there and we need to support it. We cannot
> just close eyes and pretend it doesn't exist and hope for the best.

You need to notice when you're trying to do something immpossible.

> > GFP_NOFAIL is something we very rarely use, and it's not something we
> > want to use. Furthermore, GFP_NOFAIL allocations can fail regardless of
> > this patch - e.g. if it's more than 2 pages, it's not going to be
> > GFP_NOFAIL.
> 
> We can reasonably assume we do not have any of those users in the tree
> though. We know that because we have a warning to tell us about that.
> We still have legit GFP_NOFAIL users and we can safely assume we will
> have some in the future though. And they have no way to handle the
> failure. If they did they wouldn't have used GFP_NOFAIL in the first
> place. So they do not check for NULL and they would either blow up or
> worse fail in subtle and harder to detect way.

No, because not all GFP_NOFAIL allocations are statically sized.

And the problem of the dynamic context overriding GFP_NOFAIL is more
general - if you use GFP_NOFAIL from nonblocking context (interrupt
context or preemption disabled) - the allocation has to fail, or
something even worse will happen.

Just because we don't track that with PF_MEMALLOC flags doesn't mean the
problem isn't htere.