linux-kernel - Re: [PATCH 2/4] mm/vmalloc: add support for __GFP

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Date:   Tue, 26 Oct 2021 18:28:52 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>
Subject: Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL

On Tue 26-10-21 17:48:32, Uladzislau Rezki wrote:
> > From: Michal Hocko <mhocko@...e.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@...e.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?

> 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?
-- 
Michal Hocko
SUSE Labs