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

[Muchun Song]

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/

> 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

Look at the code of gfp_to_alloc_flags().

static inline unsigned int gfp_to_alloc_flags(gfp_t gfp_mask)
{
        [...]
        if (gfp_mask & __GFP_ATOMIC) {
        /*
         * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
         * if it can't schedule.
         */
        if (!(gfp_mask & __GFP_NOMEMALLOC))
                alloc_flags |= ALLOC_HARDER;
       [...]
}

Seems to allow this operation (GFP_ATOMIC | __GFP_NOMEMALLOC).

> 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