On Mon, Feb 15, 2021 at 7:51 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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/
>
> > 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).
I also found similar users.
netdev_alloc_frag()
page_frag_alloc(GFP_ATOMIC)
__page_frag_cache_refill(GFP_ATOMIC)
alloc_pages_node(GFP_ATOMIC | __GFP_COMP | __GFP_NOWARN |
__GFP_NORETRY |
__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
