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 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

sounds like a more reasonable fit to me.

-- 
Michal Hocko
SUSE Labs