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 19:51:26, Muchun Song wrote:
> On Mon, Feb 15, 2021 at 6:33 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > 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.
> 
> A discussion about this can refer to here:
> 
> https://patchwork.kernel.org/project/linux-mm/patch/20210117151053.24600-5-songmuchun@xxxxxxxxxxxxx/

I do not see any real response to the pre-allocation argument except
that put_page can be called from an atomic context. Which might be true
in general but it is not the case for hugetlb pages. hugetlb_lock would
have to be irq safe otherwise. Also the whole operation can be scheduled
to a kworker context for a stronger allocation context.

-- 
Michal Hocko
SUSE Labs