Re: [PATCH 1/2] mm: Add memalloc_nowait_{save,restore}

[Michal Hocko <mhocko@suse.com>]

On Mon 12-08-24 20:59:53, Yafang Shao wrote:
> On Mon, Aug 12, 2024 at 7:37 PM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Mon, Aug 12, 2024 at 05:05:24PM +0800, Yafang Shao wrote:
> > > The PF_MEMALLOC_NORECLAIM flag was introduced in commit eab0af905bfc
> > > ("mm: introduce PF_MEMALLOC_NORECLAIM, PF_MEMALLOC_NOWARN"). To complement
> > > this, let's add two helper functions, memalloc_nowait_{save,restore}, which
> > > will be useful in scenarios where we want to avoid waiting for memory
> > > reclamation.
> >
> > No, forcing nowait on callee contets is just asking for trouble.
> > Unlike NOIO or NOFS this is incompatible with NOFAIL allocations
> 
> I don’t see any incompatibility in __alloc_pages_slowpath(). The
> ~__GFP_DIRECT_RECLAIM flag only ensures that direct reclaim is not
> performed, but it doesn’t prevent the allocation of pages from
> ALLOC_MIN_RESERVE, correct?

Right but this means that you just made any potential nested allocation
within the scope that is GFP_NOFAIL a busy loop essentially. Not to
mention it BUG_ON as non-sleeping GFP_NOFAIL allocations are
unsupported. I believe this is what Christoph had in mind. I am really
surprised that we even have PF_MEMALLOC_NORECLAIM in the first place!
Unsurprisingly this was merged without any review by the MM community :/
-- 
Michal Hocko
SUSE Labs