Re: [PATCH RFC] mm: allow __GFP_NOFAIL allocation up to BLK_MAX_BLOCK_SIZE to support LBS

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Oct 31, 2025 at 03:54:56PM +0000, Matthew Wilcox wrote:
> On Fri, Oct 31, 2025 at 08:52:49AM -0700, Shakeel Butt wrote:
> > After reading the background link, it seems like the actual allocation
> > will be NOFS + NOFAIL + higher_order. With NOFS, current reclaim can not
> > really reclaim any file memory (page cache). However I wonder with the
> > writeback gone from reclaim path, should we allow reclaiming clean file
> > pages even for NOFS context (need some digging).
> 
> I thought that was true yesterday morning, but I read the code and it isn't.
> 
> Look at where may_enter_fs() is called from.  It's only for folios which
> are either marked dirty or writeback.
> 

Indeed you are right.

Now for the interface to allow NOFS+NOFAIL+higher_order, I think a new
(FS specific) gfp is fine but will require some maintenance to avoid
abuse.

I am more interested in how to codify "you can reclaim one I've already
allocated". I have a different scenario where network stack keep
stealing memory from direct reclaimers and keeping them in reclaim for
long time. If we have some mechanism to allow reclaimers to get the
memory they have reclaimed (at least for some cases), I think that can
be used in both cases.