Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL

[Michal Hocko <mhocko@suse.com>]

On Tue 19-10-21 21:46:58, Uladzislau Rezki wrote:
> On Tue, Oct 19, 2021 at 01:52:07PM +0200, Michal Hocko wrote:
> > On Tue 19-10-21 13:06:49, Uladzislau Rezki wrote:
> > > > From: Michal Hocko <mhocko@suse.com>
> > > > 
> > > > Dave Chinner has mentioned that some of the xfs code would benefit from
> > > > kvmalloc support for __GFP_NOFAIL because they have allocations that
> > > > cannot fail and they do not fit into a single page.
> > > > 
> > > > The larg part of the vmalloc implementation already complies with the
> > > > given gfp flags so there is no work for those to be done. The area
> > > > and page table allocations are an exception to that. Implement a retry
> > > > loop for those.
> > > > 
> > > > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > > > ---
> > > >  mm/vmalloc.c | 6 +++++-
> > > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index 7455c89598d3..3a5a178295d1 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -2941,8 +2941,10 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > > >  	else if (!(gfp_mask & (__GFP_FS | __GFP_IO)))
> > > >  		flags = memalloc_noio_save();
> > > >  
> > > > -	ret = vmap_pages_range(addr, addr + size, prot, area->pages,
> > > > +	do {
> > > > +		ret = vmap_pages_range(addr, addr + size, prot, area->pages,
> > > >  			page_shift);
> > > > +	} while ((gfp_mask & __GFP_NOFAIL) && (ret < 0));
> > > >  
> > > >  	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
> > > >  		memalloc_nofs_restore(flags);
> > > > @@ -3032,6 +3034,8 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
> > > >  		warn_alloc(gfp_mask, NULL,
> > > >  			"vmalloc error: size %lu, vm_struct allocation failed",
> > > >  			real_size);
> > > > +		if (gfp_mask && __GFP_NOFAIL)
> > > > +			goto again;
> > > >  		goto fail;
> > > >  	}
> > > >  
> > > > -- 
> > > > 2.30.2
> > > > 
> > > I have checked the vmap code how it aligns with the __GFP_NOFAIL flag.
> > > To me it looks correct from functional point of view.
> > > 
> > > There is one place though it is kasan_populate_vmalloc_pte(). It does
> > > not use gfp_mask, instead it directly deals with GFP_KERNEL for its
> > > internal purpose. If it fails the code will end up in loping in the
> > > __vmalloc_node_range().
> > > 
> > > I am not sure how it is important to pass __GFP_NOFAIL into KASAN code.
> > > 
> > > Any thoughts about it?
> > 
> > The flag itself is not really necessary down there as long as we
> > guarantee that the high level logic doesn't fail. In this case we keep
> > retrying at __vmalloc_node_range level which should be possible to cover
> > all callers that can control gfp mask. I was thinking to put it into
> > __get_vm_area_node but that was slightly more hairy and we would be
> > losing the warning which might turn out being helpful in cases where the
> > failure is due to lack of vmalloc space or similar constrain. Btw. do we
> > want some throttling on a retry?
> > 
> I think adding kind of schedule() will not make things worse and in corner
> cases could prevent a power drain by CPU. It is important for mobile devices. 

I suspect you mean schedule_timeout here? Or cond_resched? I went with a
later for now, I do not have a good idea for how to long to sleep here.
I am more than happy to change to to a sleep though.

> As for vmap space, it can be that a user specifies a short range that does
> not contain any free area. In that case we might never return back to a caller.

This is to be expected. The caller cannot fail and if it would be
looping around vmalloc it wouldn't return anyway.

> Maybe add a good comment something like: think what you do when deal with the
> __vmalloc_node_range() and __GFP_NOFAIL?

We have a generic documentation for gfp flags and __GFP_NOFAIL is
docuemented to "The allocation could block indefinitely but will never
return with failure." We are discussing improvements for the generic
documentation in another thread [1] and we will likely extend it so I
suspect we do not have to repeat drawbacks here again.

[1] http://lkml.kernel.org/r/163184741778.29351.16920832234899124642.stgit@noble.brown

Anyway the gfp mask description and constrains for vmalloc are not
documented. I will add a new patch to fill that gap and send it as a
reply to this one
-- 
Michal Hocko
SUSE Labs