linux-kernel - Re: [PATCH] bcachefs: Switch to memalloc_flags

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Message-ID: <ZtBqwOiBThqxzckc@tiehlicka>
Date: Thu, 29 Aug 2024 14:34:08 +0200
From: Michal Hocko <mhocko@...e.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH] bcachefs: Switch to memalloc_flags_do() for vmalloc
 allocations

On Thu 29-08-24 07:55:08, Kent Overstreet wrote:
> On Thu, Aug 29, 2024 at 01:08:53PM GMT, Michal Hocko wrote:
> > On Wed 28-08-24 18:58:43, Kent Overstreet wrote:
> > > On Wed, Aug 28, 2024 at 09:26:44PM GMT, Michal Hocko wrote:
> > > > On Wed 28-08-24 15:11:19, Kent Overstreet wrote:
> > [...]
> > > > > It was decided _years_ ago that PF_MEMALLOC flags were how this was
> > > > > going to be addressed.
> > > > 
> > > > Nope! It has been decided that _some_ gfp flags are acceptable to be used
> > > > by scoped APIs. Most notably NOFS and NOIO are compatible with reclaim
> > > > modifiers and other flags so these are indeed safe to be used that way.
> > > 
> > > Decided by who?
> > 
> > Decides semantic of respective GFP flags and their compatibility with
> > others that could be nested in the scope.
> 
> Well, that's a bit of commentary, at least.
> 
> The question is which of those could properly apply to a section, not a
> callsite, and a PF_MEMALLOC_NOWAIT (similar to but not exactly the same
> as PF_MEMALLOC_NORECLAIM) would be at the top of that list since we
> already have a clear concept of sections where we're not allowed to
> sleep.

Unfortunately a lack of __GFP_DIRECT_RECLAIM means both no reclaim and
no sleeping allowed for historical reasons. GFP_NOWAIT is both used from
atomic contexts and as an optimistic allocation attempt with a heavier
fallback allocation strategy. If you want NORECLAIM semantic then this
would need to be represented by different means than __GFP_DIRECT_RECLAIM
alone.

> And that tells us how to resolve GFP_NOFAIL with other conflicting
> PF_MEMALLOC flags: GFP_NOFAIL loses.
> 
> It is a _bug_ if GFP_NOFAIL is accidentally used in a non sleepable
> context, and properly labelling those sections to the allocator would
> allow us to turn undefined behaviour into an error - _that_ would be
> turning kmalloc() into a safe interface.

If your definition of safe includes an oops or worse silent failure
then yes. Not really safe interface in my book though. E.g. (just
randomly looking at GFP_NOFAIL users) btree_paths_realloc doesn't check
the return value and if it happened to be called from such a scope it
would have blown up. That code is safe without the scope though. There
are many other callsites which do not have failure paths.
-- 
Michal Hocko
SUSE Labs