Re: [PATCH] mm/vmalloc: do not output a spurious warning when huge vmalloc() fails

Uladzislau Rezki
2023-06-06T08:17:25Z

On Tue, Jun 06, 2023 at 09:13:24AM +0200, Vlastimil Babka wrote:
>
> On 6/5/23 22:11, Lorenzo Stoakes wrote:
> > In __vmalloc_area_node() we always warn_alloc() when an allocation
> > performed by vm_area_alloc_pages() fails unless it was due to a pending
> > fatal signal.
> >
> > However, huge page allocations instigated either by vmalloc_huge() or
> > __vmalloc_node_range() (or a caller that invokes this like kvmalloc() or
> > kvmalloc_node()) always falls back to order-0 allocations if the huge page
> > allocation fails.
> >
> > This renders the warning useless and noisy, especially as all callers
> > appear to be aware that this may fallback. This has already resulted in at
> > least one bug report from a user who was confused by this (see link).
> >
> > Therefore, simply update the code to only output this warning for order-0
> > pages when no fatal signal is pending.
> >
> > Link: https://bugzilla.suse.com/show_bug.cgi?id=1211410
> > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
>
> I think there are more reports of same thing from the btrfs context, that
> appear to be a 6.3 regression
>
> https://bugzilla.kernel.org/show_bug.cgi?id=217466
> Link: https://lore.kernel.org/all/efa04d56-cd7f-6620-bca7-1df89f49bf4b@gmail.com/
>
I had a look at that report. The btrfs complains due to the
fact that a high-order page(1 << 9) can not be obtained. In the
vmalloc code we do not fall to 0-order allocator if there is
a request of getting a high-order.

I provided a patch to fallback if a high-order. A reproducer, after
applying the patch, started to get oppses in another places.

IMO, we should fallback even for high-order requests. Because it is
highly likely it can not be accomplished.

Any thoughts?

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 31ff782d368b..7a06452f7807 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2957,14 +2957,18 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
page = alloc_pages(alloc_gfp, order);
else
page = alloc_pages_node(nid, alloc_gfp, order);
+
if (unlikely(!page)) {
- if (!nofail)
- break;
+ if (nofail)
+ alloc_gfp |= __GFP_NOFAIL;

- /* fall back to the zero order allocations */
- alloc_gfp |= __GFP_NOFAIL;
- order = 0;
- continue;
+ /* Fall back to the zero order allocations. */
+ if (order || nofail) {
+ order = 0;
+ continue;
+ }
+
+ break;
}

/*
<snip>

--
Uladzislau Rezki