Re: [PATCH 06/13] mm/memory: Add dax_insert_pfn

Jan Kara
2024-06-27T11:33:38Z

On Thu 27-06-24 10:54:21, Alistair Popple wrote:
> Currently to map a DAX page the DAX driver calls vmf_insert_pfn. This
> creates a special devmap PTE entry for the pfn but does not take a
> reference on the underlying struct page for the mapping. This is
> because DAX page refcounts are treated specially, as indicated by the
> presence of a devmap entry.
>
> To allow DAX page refcounts to be managed the same as normal page
> refcounts introduce dax_insert_pfn. This will take a reference on the
> underlying page much the same as vmf_insert_page, except it also
> permits upgrading an existing mapping to be writable if
> requested/possible.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>

Overall this looks good to me. Some comments below.

> ---
> include/linux/mm.h | 4 ++-
> mm/memory.c | 79 ++++++++++++++++++++++++++++++++++++++++++-----
> 2 files changed, 76 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9a5652c..b84368b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1080,6 +1080,8 @@ int vma_is_stack_for_current(struct vm_area_struct *vma);
> struct mmu_gather;
> struct inode;
>
> +extern void prep_compound_page(struct page *page, unsigned int order);
> +

You don't seem to use this function in this patch?

> diff --git a/mm/memory.c b/mm/memory.c
> index ce48a05..4f26a1f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1989,14 +1989,42 @@ static int validate_page_before_insert(struct page *page)
> }
>
> static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
> - unsigned long addr, struct page *page, pgprot_t prot)
> + unsigned long addr, struct page *page, pgprot_t prot, bool mkwrite)
> {
> struct folio *folio = page_folio(page);
> + pte_t entry = ptep_get(pte);
>
> - if (!pte_none(ptep_get(pte)))
> + if (!pte_none(entry)) {
> + if (mkwrite) {
> + /*
> + * For read faults on private mappings the PFN passed
> + * in may not match the PFN we have mapped if the
> + * mapped PFN is a writeable COW page. In the mkwrite
> + * case we are creating a writable PTE for a shared
> + * mapping and we expect the PFNs to match. If they
> + * don't match, we are likely racing with block
> + * allocation and mapping invalidation so just skip the
> + * update.
> + */
> + if (pte_pfn(entry) != page_to_pfn(page)) {
> + WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
> + return -EFAULT;
> + }
> + entry = maybe_mkwrite(entry, vma);
> + entry = pte_mkyoung(entry);
> + if (ptep_set_access_flags(vma, addr, pte, entry, 1))
> + update_mmu_cache(vma, addr, pte);
> + return 0;
> + }
> return -EBUSY;

If you do this like:

if (!mkwrite)
return -EBUSY;

You can reduce indentation of the big block and also making the flow more
obvious...

> + }
> +
> /* Ok, finally just insert the thing.. */
> folio_get(folio);
> + if (mkwrite)
> + entry = maybe_mkwrite(mk_pte(page, prot), vma);
> + else
> + entry = mk_pte(page, prot);

I'd prefer:

entry = mk_pte(page, prot);
if (mkwrite)
entry = maybe_mkwrite(entry, vma);

but I don't insist. Also insert_pfn() additionally has pte_mkyoung() and
pte_mkdirty(). Why was it left out here?

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR