On Wed, Jun 28, 2017 at 04:01:48PM -0600, Ross Zwisler wrote:
> To be able to use the common 4k zero page in DAX we need to have our PTE
> fault path look more like our PMD fault path where a PTE entry can be
> marked as dirty and writeable as it is first inserted, rather than waiting
> for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
>
> Right now we can rely on having a dax_pfn_mkwrite() call because we can
> distinguish between these two cases in do_wp_page():
>
> case 1: 4k zero page => writable DAX storage
> case 2: read-only DAX storage => writeable DAX storage
>
> This distinction is made by via vm_normal_page(). vm_normal_page() returns
> false for the common 4k zero page, though, just as it does for DAX ptes.
> Instead of special casing the DAX + 4k zero page case, we will simplify our
> DAX PTE page fault sequence so that it matches our DAX PMD sequence, and
> get rid of dax_pfn_mkwrite() completely.
>
> This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
> and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set insert_pfn()
> will do the work that was previously done by wp_page_reuse() as part of the
> dax_pfn_mkwrite() call path.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
> include/linux/mm.h | 2 ++
> mm/memory.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++-----
> 2 files changed, 54 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6f543a4..096052f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2293,6 +2293,8 @@ int vm_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
> unsigned long pfn, pgprot_t pgprot);
> int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
> pfn_t pfn);
> +int vm_insert_mixed_mkwrite(struct vm_area_struct *vma, unsigned long addr,
> + pfn_t pfn);
> int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long len);
>
>
> diff --git a/mm/memory.c b/mm/memory.c
> index bb11c47..de4aa71 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1646,7 +1646,7 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
> EXPORT_SYMBOL(vm_insert_page);
>
> static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
> - pfn_t pfn, pgprot_t prot)
> + pfn_t pfn, pgprot_t prot, bool mkwrite)
> {
> struct mm_struct *mm = vma->vm_mm;
> int retval;
> @@ -1658,14 +1658,26 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
> if (!pte)
> goto out;
> retval = -EBUSY;
> - if (!pte_none(*pte))
> - goto out_unlock;
> + if (!pte_none(*pte)) {
> + if (mkwrite) {
> + entry = *pte;
> + goto out_mkwrite;
> + } else
> + goto out_unlock;
> + }
>
> /* Ok, finally just insert the thing.. */
> if (pfn_t_devmap(pfn))
> entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
> else
> entry = pte_mkspecial(pfn_t_pte(pfn, prot));
> +
> +out_mkwrite:
> + if (mkwrite) {
> + entry = pte_mkyoung(entry);
> + entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> + }
> +
> set_pte_at(mm, addr, pte, entry);
> update_mmu_cache(vma, addr, pte); /* XXX: why not for insert_page? */
>
> @@ -1736,7 +1748,8 @@ int vm_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
>
> track_pfn_insert(vma, &pgprot, __pfn_to_pfn_t(pfn, PFN_DEV));
>
> - ret = insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot);
> + ret = insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot,
> + false);
>
> return ret;
> }
> @@ -1772,10 +1785,44 @@ int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
> page = pfn_to_page(pfn_t_to_pfn(pfn));
> return insert_page(vma, addr, page, pgprot);
> }
> - return insert_pfn(vma, addr, pfn, pgprot);
> + return insert_pfn(vma, addr, pfn, pgprot, false);
> }
> EXPORT_SYMBOL(vm_insert_mixed);
>
> +int vm_insert_mixed_mkwrite(struct vm_area_struct *vma, unsigned long addr,
> + pfn_t pfn)
> +{
> + pgprot_t pgprot = vma->vm_page_prot;
> +
> + BUG_ON(!(vma->vm_flags & VM_MIXEDMAP));
> +
> + if (addr < vma->vm_start || addr >= vma->vm_end)
> + return -EFAULT;
> +
> + track_pfn_insert(vma, &pgprot, pfn);
> +
> + /*
> + * If we don't have pte special, then we have to use the pfn_valid()
> + * based VM_MIXEDMAP scheme (see vm_normal_page), and thus we *must*
> + * refcount the page if pfn_valid is true (hence insert_page rather
> + * than insert_pfn). If a zero_pfn were inserted into a VM_MIXEDMAP
> + * without pte special, it would there be refcounted as a normal page.
> + */
> + if (!HAVE_PTE_SPECIAL && !pfn_t_devmap(pfn) && pfn_t_valid(pfn)) {
> + struct page *page;
> +
> + /*
> + * At this point we are committed to insert_page()
> + * regardless of whether the caller specified flags that
> + * result in pfn_t_has_page() == false.
> + */
> + page = pfn_to_page(pfn_t_to_pfn(pfn));
> + return insert_page(vma, addr, page, pgprot);
> + }
> + return insert_pfn(vma, addr, pfn, pgprot, true);
> +}
> +EXPORT_SYMBOL(vm_insert_mixed_mkwrite);
Hi Ross,
vm_insert_mixed_mkwrite() is same as vm_insert_mixed() except this sets
write parameter to inser_pfn() true. Will it make sense to just add
mkwrite parameter to vm_insert_mixed() and not add a new helper function.
(like insert_pfn()).
Vivek
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