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Date:   Wed, 27 Oct 2021 08:46:50 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     Linux Memory Management List <linux-mm@kvack.org>,
        Dave Chinner <david@fromorbit.com>, Neil Brown <neilb@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>,
        Jeff Layton <jlayton@kernel.org>
Subject: Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL
Message-ID: <YXj12kfAOcIeKFb+@dhcp22.suse.cz>
References: <20211025150223.13621-1-mhocko@kernel.org>
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On Tue 26-10-21 21:33:15, Uladzislau Rezki wrote:
> 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..

Yes, the page allocator doesn't warn either and I would like to keep
this in sync.

> 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?

OK, I will change that.
-- 
Michal Hocko
SUSE Labs