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

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Message-ID: <rwqusvtkwzbr2pc2hwmt2lkpffzivrlaw3xfrnrqxze6wmpsex@s3eavvieveld>
Date: Mon, 26 Aug 2024 15:42:59 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
Cc: Michal Hocko <mhocko@...nel.org>, 
	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 1/2] bcachefs: do not use PF_MEMALLOC_NORECLAIM

On Mon, Aug 26, 2024 at 08:41:42PM GMT, Matthew Wilcox wrote:
> On Mon, Aug 26, 2024 at 03:39:47PM -0400, Kent Overstreet wrote:
> > Given the amount of plumbing required here, it's clear that passing gfp
> > flags is the less safe way of doing it, and this really does belong in
> > the allocation context.
> > 
> > Failure to pass gfp flags correctly (which we know is something that
> > happens today, e.g. vmalloc -> pte allocation) means you're introducing
> > a deadlock.
> 
> The problem with vmalloc is that the page table allocation _doesn't_
> take a GFP parameter.

yeah, I know. I posted patches to plumb it through, which were nacked by
Linus.

And we're trying to get away from passing gfp flags directly, are we
not? I just don't buy the GFP_NOFAIL unsafety argument.