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

Alistair Popple
2024-09-06T06:25:57Z

Jan Kara <jack@suse.cz> writes:

> 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?

Thanks, bad rebase splitting this up. It belongs later in the series.

>> 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...

Good idea.

>> + }
>> +
>> /* 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?

An oversight by me, thanks for pointing it out!

> Honza