Re: [RFC PATCH v2] mm/vmalloc: fix incorrect __vmap_pages_range_noflush() if vm_area_alloc_pages() from high order fallback to order0

Barry Song
2024-07-26T05:29:56Z

On Fri, Jul 26, 2024 at 5:04 PM Hailong Liu <hailong.liu@oppo.com> wrote:
>
> On Fri, 26. Jul 12:00, Hailong Liu wrote:
> > On Fri, 26. Jul 10:31, Baoquan He wrote:
> > [...]
> > > > The logic of this patch is somewhat similar to my first one. If high order
> > > > allocation fails, it will go normal mapping.
> > > >
> > > > However I also save the fallback position. The ones before this position are
> > > > used for huge mapping, the ones >= position for normal mapping as Barry said.
> > > > "support the combination of PMD and PTE mapping". this will take some
> > > > times as it needs to address the corner cases and do some tests.
> > >
> > > Hmm, we may not need to worry about the imperfect mapping. Currently
> > > there are two places setting VM_ALLOW_HUGE_VMAP: __kvmalloc_node_noprof()
> > > and vmalloc_huge().
> > >
> > > For vmalloc_huge(), it's called in below three interfaces which are all
> > > invoked during boot. Basically they can succeed to get required contiguous
> > > physical memory. I guess that's why Tangquan only spot this issue on kvmalloc
> > > invocation when the required size exceeds e.g 2M. For kvmalloc_node(),
> > > we have told that in the code comment above __kvmalloc_node_noprof(),
> > > it's a best effort behaviour.
> > >
> > Take a __vmalloc_node_range(2.1M, VM_ALLOW_HUGE_VMAP) as a example.
> > because the align requirement of huge. the real size is 4M.
> > if allocation first order-9 successfully and the next failed. becuase the
> > fallback, the layout out pages would be like order9 - 512 * order0
> > order9 support huge mapping, but order0 not.
> > with the patch above, would call vmap_small_pages_range_noflush() and do normal
> > mapping, the huge mapping would not exist.
> >
> > > mm/mm_init.c <<alloc_large_system_hash>>
> > > table = vmalloc_huge(size, gfp_flags);
> > > net/ipv4/inet_hashtables.c <<inet_pernet_hashinfo_alloc>>
> > > new_hashinfo->ehash = vmalloc_huge(ehash_entries * sizeof(struct inet_ehash_bucket),
> > > net/ipv4/udp.c <<udp_pernet_table_alloc>>
> > > udptable->hash = vmalloc_huge(hash_entries * 2 * sizeof(struct udp_hslot)
> > >
> > > Maybe we should add code comment or document to notice people that the
> > > contiguous physical pages are not guaranteed for vmalloc_huge() if you
> > > use it after boot.
> > >
> > > >
> > > > IMO, the draft can fix the current issue, it also does not have significant side
> > > > effects. Barry, what do you think about this patch? If you think it's okay,
> > > > I will split this patch into two: one to remove the VM_ALLOW_HUGE_VMAP and the
> > > > other to address the current mapping issue.
> > > >
> > > > --
> > > > help you, help me,
> > > > Hailong.
> > > >
> > >
> > >
> I check the code, the issue only happen in gfp_mask with __GFP_NOFAIL and
> fallback to order 0, actuaally without this commit
> e9c3cda4d86e ("mm, vmalloc: fix high order __GFP_NOFAIL allocations")
> if __vmalloc_area_node allocation failed, it will goto fail and try order-0.
>
> fail:
> if (shift > PAGE_SHIFT) {
> shift = PAGE_SHIFT;
> align = real_align;
> size = real_size;
> goto again;
> }
>
> So do we really need fallback to order-0 if nofail?

Good catch, this is what I missed. I feel we can revert Michal's fix.
And just remove __GFP_NOFAIL bit when we are still allocating
by high-order. When "goto again" happens, we will allocate by
order-0, in this case, we keep the __GFP_NOFAIL.

> >
> > --
> > help you, help me,
> > Hailong.

Thanks
Barry