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

[Michal Hocko]

On Fri 20-11-20 17:37:09, Muchun Song wrote:
> On Fri, Nov 20, 2020 at 5:28 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Fri 20-11-20 16:51:59, Muchun Song wrote:
> > > On Fri, Nov 20, 2020 at 4:11 PM Michal Hocko <mhocko@xxxxxxxx> 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
> 
> We want to free the HugeTLB page to the buddy allocator, but before that,
> we need to allocate some pages as vmemmap pages, so here we cannot
> handle allocation failures.

Why cannot you simply refuse to shrink the pool size?

> I think that we should replace the
> __GFP_RETRY_MAYFAIL to __GFP_NOFAIL.
> 
> GFP_KERNEL | __GFP_NOFAIL | __GFP_HIGH
> 
> This meets our needs here. Thanks.

Please read again my concern about the disruptive behavior or explain
why it is desirable.

-- 
Michal Hocko
SUSE Labs