Re: [PATCH 1/2] bcachefs: do not use PF_MEMALLOC_NORECLAIM

[Kent Overstreet <kent.overstreet@linux.dev>]

On Mon, Aug 26, 2024 at 10:47:12AM GMT, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.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.

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.