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

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Message-ID: <20240726050356.ludmpxfee6erlxxt@oppo.com>
Date: Fri, 26 Jul 2024 13:03:56 +0800
From: Hailong Liu <hailong.liu@...o.com>
To: Baoquan He <bhe@...hat.com>
CC: Barry Song <21cnbao@...il.com>, Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
	Lorenzo Stoakes <lstoakes@...il.com>, Vlastimil Babka <vbabka@...e.cz>,
	Michal Hocko <mhocko@...e.com>, Matthew Wilcox <willy@...radead.org>,
	Tangquan Zheng <zhengtangquan@...o.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2] mm/vmalloc: fix incorrect
 __vmap_pages_range_noflush() if vm_area_alloc_pages() from high order
 fallback to order0

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

--
help you, help me,
Hailong.