Re: [External] Re: [PATCH v15 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Michal Hocko]

On Mon 15-02-21 18:05:06, Muchun Song wrote:
> On Fri, Feb 12, 2021 at 11:32 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
[...]
> > > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > > +{
> > > +     int ret;
> > > +     unsigned long vmemmap_addr = (unsigned long)head;
> > > +     unsigned long vmemmap_end, vmemmap_reuse;
> > > +
> > > +     if (!free_vmemmap_pages_per_hpage(h))
> > > +             return 0;
> > > +
> > > +     vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > > +     vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > > +     vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > > +
> > > +     /*
> > > +      * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > > +      * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > > +      * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > > +      * When a HugeTLB page is freed to the buddy allocator, previously
> > > +      * discarded vmemmap pages must be allocated and remapping.
> > > +      */
> > > +     ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > > +                               GFP_ATOMIC | __GFP_NOWARN | __GFP_THISNODE);
> >
> > I do not think that this is a good allocation mode. GFP_ATOMIC is a non
> > sleeping allocation and a medium memory pressure might cause it to
> > fail prematurely. I do not think this is really an atomic context which
> > couldn't afford memory reclaim. I also do not think we want to grant
> 
> Because alloc_huge_page_vmemmap is called under hugetlb_lock
> now. So using GFP_ATOMIC indeed makes the code more simpler.

You can have a preallocated list of pages prior taking the lock.
Moreover do we want to manipulate vmemmaps from under spinlock in
general. I have to say I have missed that detail when reviewing. Need to
think more.

> From the document of the kernel, I learned that __GFP_NOMEMALLOC
> can be used to explicitly forbid access to emergency reserves. So if
> we do not want to use the reserve memory. How about replacing it to
> 
> GFP_ATOMIC | __GFP_NOMEMALLOC | __GFP_NOWARN | __GFP_THISNODE

The whole point of GFP_ATOMIC is to grant access to memory reserves so
the above is quite dubious. If you do not want access to memory reserves
then use GFP_NOWAIT instead. But failures are much more easier to happen
then.

NOMEMALLOC is meant to be used from paths which are allowed to consume
memory reserves - e.g. when invoked from the memory reclaim path.
-- 
Michal Hocko
SUSE Labs