Re: [PATCH] mm: allow __GFP_RETRY_MAYFAIL in vmalloc

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Feb 23, 2026 at 11:08:02PM +0100, Michal Hocko wrote:
> On Mon 23-02-26 20:25:38, Mikulas Patocka wrote:
> > 
> > 
> > On Mon, 23 Feb 2026, Vishal Moola (Oracle) wrote:
> > 
> > > On Thu, Feb 12, 2026 at 05:33:30PM +0100, Mikulas Patocka wrote:
> > > > The commit 07003531e03c8 ("mm/vmalloc: warn on invalid vmalloc gfp
> > > > flags") breaks the device mapper VDO target. The VDO target calls vmalloc
> > > > with __GFP_RETRY_MAYFAIL and this flag is not in the mask of allowed
> > > > flags.
> > > > 
> > > > There is no reason why vmalloc couldn't support __GFP_RETRY_MAYFAIL, so 
> > > > let's add this flag to GFP_VMALLOC_SUPPORTED.
> > > 
> > > My only skepticism about this comes from the line in the
> > > vmalloc_node_range() doc: 
> > > "and %__GFP_RETRY_MAYFAIL are not supported."
> > > 
> > > I myself don't know why that may be. Could you elaborate on if/why the
> > > doc is wrong please?
> > 
> > This statement was added by Michal Hocko in the commit 
> > b7d90e7a5ea8d64e668d5685925900d33d3884d5. Michal, could you explain why do 
> > you think that __GFP_RETRY_MAYFAIL is not supported?
> 
> The problem with __GFP_RETRY_MAYFAIL is that it cannot be fully
> supported. While pages that back the allocation can be easily made aware
> of this failure mode there are page table allocations which are
> hardocded GFP_KERNEL and there is no sensible way to extend the API to
> change that (as we have learned several time over years).
> 
> > The VDO module needs to allocate large amounts of memory and it doesn't 
> > want to trigger the OOM killer (which would kill some innocent task and 
> > wouldn't solve the out of memory condition at all), so I think that 
> > __GFP_RETRY_MAYFAIL is appropriate.
> 
> Understood. But as said the very page table allocation could be the
> trigger for the unwanted OOM. The same applies to __GFP_NORETRY
> unfortunately as well. vmalloc has just recently gained support of
> GFP_NOWAIT allocation mode, though. This will make the allocation
> failure much more likely though so I am not entirely sure this is a
> proper solution for your problem.
>
Yes, the page-table manipulation entries are hard-coded and it looks
like it is the last path which is not wired properly with gfp-flags.

Since we grow PTEs and never release it might not be a big issue for
the __GFP_RETRY_MAYFAIL usage. But it is still not valid in noted path.

--
Uladzislau Rezki