Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Oct 26, 2021 at 06:28:52PM +0200, Michal Hocko wrote:
> On Tue 26-10-21 17:48:32, 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.
> > >
> > > Add a short sleep before retrying. 1 jiffy is a completely random
> > > timeout. Ideally the retry would wait for an explicit event - e.g.
> > > a change to the vmalloc space change if the failure was caused by
> > > the space fragmentation or depletion. But there are multiple different
> > > reasons to retry and this could become much more complex. Keep the retry
> > > simple for now and just sleep to prevent from hogging CPUs.
> > >
> > > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > > ---
> > >  mm/vmalloc.c | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index c6cc77d2f366..602649919a9d 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -2941,8 +2941,12 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > >         else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
> > >                 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);
> > > +               if (ret < 0)
> > > +                       schedule_timeout_uninterruptible(1);
> > > +       } while ((gfp_mask & __GFP_NOFAIL) && (ret < 0));
> > >
> > 
> > 1.
> > After that change a below code:
> > 
> > <snip>
> > if (ret < 0) {
> >     warn_alloc(orig_gfp_mask, NULL,
> >         "vmalloc error: size %lu, failed to map pages",
> >         area->nr_pages * PAGE_SIZE);
> >     goto fail;
> > }
> > <snip>
> > 
> > does not make any sense anymore.
> 
> Why? Allocations without __GFP_NOFAIL can still fail, no?
> 
Right. I meant one thing but wrote slightly differently. In case of
vmap_pages_range() fails(if __GFP_NOFAIL is set) should we emit any
warning message? Because either we can recover on a future iteration
or it stuck there infinitely so a user does not understand what happened.
From the other hand this is how __GFP_NOFAIL works, hm..

Another thing, i see that schedule_timeout_uninterruptible(1) is invoked
for all cases even when __GFP_NOFAIL is not set, in that scenario we do
not want to wait, instead we should return back to a caller asap. Or am
i missing something here?

> > 2.
> > Can we combine two places where we handle __GFP_NOFAIL into one place?
> > That would look like as more sorted out.
> 
> I have to admit I am not really fluent at vmalloc code so I wanted to
> make the code as simple as possible. How would I unwind all the allocated
> memory (already allocated as GFP_NOFAIL) before retrying at
> __vmalloc_node_range (if that is what you suggest). And isn't that a
> bit wasteful?
> 
> Or did you have anything else in mind?
>
It depends on how often all this can fail. But let me double check if
such combining is easy.

--
Vlad Rezki