On Fri, Feb 11, 2022 at 8:37 PM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
>
> On 2/11/22 07:54, Muchun Song wrote:
> > On Fri, Feb 11, 2022 at 3:34 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
> > [...]
> >> pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
> >> - struct vmem_altmap *altmap)
> >> + struct vmem_altmap *altmap,
> >> + struct page *block)
> >
> > Why not use the name of "reuse" instead of "block"?
> > Seems like "reuse" is more clear.
> >
> Good idea, let me rename that to @reuse.
>
> >> {
> >> pte_t *pte = pte_offset_kernel(pmd, addr);
> >> if (pte_none(*pte)) {
> >> pte_t entry;
> >> void *p;
> >>
> >> - p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
> >> - if (!p)
> >> - return NULL;
> >> + if (!block) {
> >> + p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
> >> + if (!p)
> >> + return NULL;
> >> + } else {
> >> + /*
> >> + * When a PTE/PMD entry is freed from the init_mm
> >> + * there's a a free_pages() call to this page allocated
> >> + * above. Thus this get_page() is paired with the
> >> + * put_page_testzero() on the freeing path.
> >> + * This can only called by certain ZONE_DEVICE path,
> >> + * and through vmemmap_populate_compound_pages() when
> >> + * slab is available.
> >> + */
> >> + get_page(block);
> >> + p = page_to_virt(block);
> >> + }
> >> entry = pfn_pte(__pa(p) >> PAGE_SHIFT, PAGE_KERNEL);
> >> set_pte_at(&init_mm, addr, pte, entry);
> >> }
> >> @@ -609,7 +624,8 @@ pgd_t * __meminit vmemmap_pgd_populate(unsigned long addr, int node)
> >> }
> >>
> >> static int __meminit vmemmap_populate_address(unsigned long addr, int node,
> >> - struct vmem_altmap *altmap)
> >> + struct vmem_altmap *altmap,
> >> + struct page *reuse, struct page **page)
> >
> > We can remove the last argument (struct page **page) if we change
> > the return type to "pte_t *". More simple, don't you think?
> >
>
> Hmmm, perhaps it is simpler, specially provided the only error code is ENOMEM.
>
> Albeit perhaps what we want is a `struct page *` rather than a pte.
The caller can extract `struct page` from a pte.
[...]
> >> - if (vmemmap_populate(start, end, nid, altmap))
> >> + if (pgmap && pgmap_vmemmap_nr(pgmap) > 1 && !altmap)
> >
> > Should we add a judgment like "is_power_of_2(sizeof(struct page))" since
> > this optimization is only applied when the size of the struct page does not
> > cross page boundaries?
>
> Totally miss that -- let me make that adjustment.
>
> Can I ask which architectures/conditions this happens?
E.g. arm64 when !CONFIG_MEMCG.
Thanks.
