Re: [PATCH 2/2] mm: introduce PF_MEMALLOC_NOWARN

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

On Sun, Jan 28, 2024 at 04:45:32PM +0100, Michal Hocko wrote:
> On Fri 26-01-24 17:07:56, Kent Overstreet wrote:
> > If we're using PF_MEMALLOC, we might have a fallback and might not to
> > warn about a failing allocation - thus we need a PF_* equivalent of
> > __GFP_NOWARN.
> 
> Could you be more specific about the user? Is this an allocation from
> the reclaim path or an explicit PF_MEMALLOC one? It would be also really
> helpful to explain why GFP_NOWARN cannot be used directly.

Explicit PF_MEMALLOC.

It's for a call to alloc_inode(), which doesn't take gfp flags, and
plumbing it would require modifying a s_ops callback and would touch
every filesystem - but we want to get away from gfp flags anyways :)

More specifically, the code where I'm using it is doing a "try
GFP_NOWAIT first; if that fails drop locks and do GFP_KERNEL" dance;
it's part of a cleanup for some weird lifetime stuff related to
fs/inode.c.

#define memalloc_flags_do(_flags, _do)						\
({										\
	unsigned _saved_flags = memalloc_flags_save(_flags);			\
	typeof(_do) _ret = _do;							\
	memalloc_noreclaim_restore(_saved_flags);				\
	_ret;									\
})

/*
 * Allocate a new inode, dropping/retaking btree locks if necessary:
 */
static struct bch_inode_info *bch2_new_inode(struct btree_trans *trans)
{
	struct bch_fs *c = trans->c;

	struct bch_inode_info *inode =
		memalloc_flags_do(PF_MEMALLOC|PF_MEMALLOC_NOWARN, to_bch_ei(new_inode(c->vfs_sb)));

	if (unlikely(!inode)) {
		int ret = drop_locks_do(trans, (inode = to_bch_ei(new_inode(c->vfs_sb))) ? 0 : -ENOMEM);
		if (ret && inode)
			discard_new_inode(&inode->v);
		if (ret)
			return ERR_PTR(ret);
	}

	return inode;
}