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

[Uladzislau Rezki <urezki@gmail.com>]

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