Re: [RFC PATCH] mm/vmalloc: fix vmalloc which may return null if called with __GFP_NOFAIL

Hailong Liu
2024-05-09T01:31:05Z

On Wed, 08. May 23:40, Gao Xiang wrote:
>
>
> On 2024/5/8 23:31, Hailong Liu wrote:
> > On Wed, 08. May 23:10, Gao Xiang wrote:
> > > Hi,
> > >
> > > On 2024/5/8 22:43, Hailong Liu wrote:
> > > > On Wed, 08. May 21:41, Gao Xiang wrote:
> > > > >
> > > > > +Cc Michal,
> > > > >
> > > > > On 2024/5/8 20:58, hailong.liu@oppo.com wrote:
> > > > > > From: "Hailong.Liu" <hailong.liu@oppo.com>
> > > > > >
> > > > > > Commit a421ef303008 ("mm: allow !GFP_KERNEL allocations for kvmalloc")
> > > > > > includes support for __GFP_NOFAIL, but it presents a conflict with
> > > > > > commit dd544141b9eb ("vmalloc: back off when the current task is
> > > > > > OOM-killed"). A possible scenario is as belows:
> > > > > >
> > > > > > process-a
> > > > > > kvcalloc(n, m, GFP_KERNEL | __GFP_NOFAIL)
> > > > > > __vmalloc_node_range()
> > > > > > __vmalloc_area_node()
> > > > > > vm_area_alloc_pages()
> > > > > > --> oom-killer send SIGKILL to process-a
> > > > > > if (fatal_signal_pending(current)) break;
> > > > > > --> return NULL;
> > > > > >
> > > > > > to fix this, do not check fatal_signal_pending() in vm_area_alloc_pages()
> > > > > > if __GFP_NOFAIL set.
> > > > > >
> > > > > > Reported-by: Oven <liyangouwen1@oppo.com>
> > > > > > Signed-off-by: Hailong.Liu <hailong.liu@oppo.com>
> > > > >
> > > > > Why taging this as RFC here? It seems a corner-case fix of
> > > > > commit a421ef303008
> > > > >
> > > > > Thanks,
> > > > > Gao Xiang
> > > > >
> > > >
> > > > Hi Gao Xiang:
> > > >
> > > > RFC here to wait for a better way to handle this case :).
> > > > IMO, if vmalloc support __GFP_NOFAIL it should not return
> > > > null even system is deadlock on memory.
> > >
> > > The starting point is that kmalloc doesn't support __GFP_NOFAIL
> > > if order > 1 (even for very short temporary uses), see:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/page_alloc.c?h=v6.8#n2896
> > >
> > > but it is possible if we have such page pointer array (since two
> > > (order-1) pages can only keep 1024 8-byte entries, it can happen
> > > if compression ratios are high), and kvmalloc(__GFP_NOFAIL) has
> > > already been supported for almost two years, it will fallback to
> > > order-0 allocation as described in commit e9c3cda4d86e
> > > ("mm, vmalloc: fix high order __GFP_NOFAIL allocations").
> > >
> > > With my limited understanding, I'm not sure why it can cause
> > > deadlock here since it will fallback to order-0 allocation then,
> > > and such allocation is just for short temporary uses again
> > > because kmalloc doesn't support order > 1 short memory
> > > allocation strictly.
> > >
> >
> > deadlock on memory meands there is a memory leak causing
> > system to be unable to allocate memory not actual
> > *deadlock*.
>
> Where is memory leak? If it's caused by kvmalloc(__GFP_NOFAIL)
> callers, then it's bugs of callers and we should fix the callers.
>
> Also why kmalloc(__GFP_NOFAIL) (for example, also order-0
> allocation) differs?
>
> Thanks,
> Gao Xiang
>

I’m not suggesting that erofs would cause a memleak. What I mean is
that if kvmalloc is invoked with __GFP_NOFAIL, it must ensure a non-NULL
return, even in scenarios where memory leaks caused by other processes
result in the inability to allocate a page. In such a situation, it
should result in “Kernel panic - not syncing: System is deadlocked
on memory”.
>
> >
> > > Thanks,
> > > Gao Xiang
> > >
> >
> > --
> >
> > Best Regards,
> > Hailong.

--
Best Regards,
Hailong.