From: Michal Hocko <mhocko@suse.com>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: 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: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL
Date: Wed, 20 Oct 2021 10:25:06 +0200 [thread overview]
Message-ID: <YW/SYl/ZKp7W60mg@dhcp22.suse.cz> (raw)
In-Reply-To: <20211019194658.GA1787@pc638.lan>
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
next prev parent reply other threads:[~2021-10-20 8:25 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-18 11:47 [RFC 0/3] extend vmalloc support for constrained allocations Michal Hocko
2021-10-18 11:47 ` [RFC 1/3] mm/vmalloc: alloc GFP_NO{FS,IO} for vmalloc Michal Hocko
2021-10-19 0:44 ` NeilBrown
2021-10-19 6:59 ` Michal Hocko
2021-10-18 11:47 ` [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL Michal Hocko
2021-10-18 16:48 ` Michal Hocko
2021-10-19 11:06 ` Uladzislau Rezki
2021-10-19 11:52 ` Michal Hocko
2021-10-19 19:46 ` Uladzislau Rezki
2021-10-20 8:25 ` Michal Hocko [this message]
2021-10-20 9:18 ` Michal Hocko
2021-10-20 13:54 ` Uladzislau Rezki
2021-10-20 14:06 ` Michal Hocko
2021-10-20 14:29 ` Uladzislau Rezki
2021-10-20 14:53 ` Michal Hocko
2021-10-20 15:00 ` Uladzislau Rezki
2021-10-20 19:24 ` Uladzislau Rezki
2021-10-21 8:56 ` Michal Hocko
2021-10-21 10:13 ` NeilBrown
2021-10-21 10:27 ` Michal Hocko
2021-10-21 10:40 ` Uladzislau Rezki
2021-10-21 22:49 ` NeilBrown
2021-10-22 8:18 ` Michal Hocko
2021-10-25 9:48 ` Uladzislau Rezki
2021-10-25 11:20 ` Michal Hocko
2021-10-25 14:30 ` Uladzislau Rezki
2021-10-25 14:56 ` Michal Hocko
2021-10-25 23:50 ` NeilBrown
2021-10-26 7:16 ` Michal Hocko
2021-10-26 10:24 ` NeilBrown
2021-10-26 14:25 ` Uladzislau Rezki
2021-10-26 14:43 ` Michal Hocko
2021-10-26 15:40 ` Uladzislau Rezki
2021-10-20 8:25 ` [PATCH] mm/vmalloc: be more explicit about supported gfp flags Michal Hocko
2021-10-18 11:47 ` [RFC 3/3] mm: allow !GFP_KERNEL allocations for kvmalloc Michal Hocko
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