Re: [External] Re: [PATCH v5 11/21] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page

From: Michal Hocko <mhocko@suse.com>
To: Muchun Song <songmuchun@bytedance.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
	dave.hansen@linux.intel.com, luto@kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	viro@zeniv.linux.org.uk,
	Andrew Morton <akpm@linux-foundation.org>,
	paulmck@kernel.org, mchehab+huawei@kernel.org,
	pawan.kumar.gupta@linux.intel.com,
	Randy Dunlap <rdunlap@infradead.org>,
	oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
	Mina Almasry <almasrymina@google.com>,
	David Rientjes <rientjes@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Oscar Salvador <osalvador@suse.de>,
	"Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
	Xiongchun duan <duanxiongchun@bytedance.com>,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH v5 11/21] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page
Date: Fri, 20 Nov 2020 10:28:26 +0100	[thread overview]
Message-ID: <20201120092826.GL3200@dhcp22.suse.cz> (raw)
In-Reply-To: <CAMZfGtWVxCPpL7=0dfHa7_qtakmGDMLP0twWoyM=gVou=HRmEg@mail.gmail.com>

On Fri 20-11-20 16:51:59, Muchun Song wrote:
> On Fri, Nov 20, 2020 at 4:11 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Fri 20-11-20 14:43:15, Muchun Song wrote:
> > [...]
> > > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > > index eda7e3a0b67c..361c4174e222 100644
> > > --- a/mm/hugetlb_vmemmap.c
> > > +++ b/mm/hugetlb_vmemmap.c
> > > @@ -117,6 +117,8 @@
> > >  #define RESERVE_VMEMMAP_NR           2U
> > >  #define RESERVE_VMEMMAP_SIZE         (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
> > >  #define TAIL_PAGE_REUSE                      -1
> > > +#define GFP_VMEMMAP_PAGE             \
> > > +     (GFP_KERNEL | __GFP_NOFAIL | __GFP_MEMALLOC)
> >
> > This is really dangerous! __GFP_MEMALLOC would allow a complete memory
> > depletion. I am not even sure triggering the OOM killer is a reasonable
> > behavior. It is just unexpected that shrinking a hugetlb pool can have
> > destructive side effects. I believe it would be more reasonable to
> > simply refuse to shrink the pool if we cannot free those pages up. This
> > sucks as well but it isn't destructive at least.
> 
> I find the instructions of __GFP_MEMALLOC from the kernel doc.
> 
> %__GFP_MEMALLOC allows access to all memory. This should only be used when
> the caller guarantees the allocation will allow more memory to be freed
> very shortly.
> 
> Our situation is in line with the description above. We will free a HugeTLB page
> to the buddy allocator which is much larger than that we allocated shortly.

Yes that is a part of the description. But read it in its full entirety.
 * %__GFP_MEMALLOC allows access to all memory. This should only be used when
 * the caller guarantees the allocation will allow more memory to be freed
 * very shortly e.g. process exiting or swapping. Users either should
 * be the MM or co-ordinating closely with the VM (e.g. swap over NFS).
 * Users of this flag have to be extremely careful to not deplete the reserve
 * completely and implement a throttling mechanism which controls the
 * consumption of the reserve based on the amount of freed memory.
 * Usage of a pre-allocated pool (e.g. mempool) should be always considered
 * before using this flag.

GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_HIGH

sounds like a more reasonable fit to me.

-- 
Michal Hocko
SUSE Labs