Re: [PATCH v3 02/10] fsdax: Factor helper: dax_fault_actor()

Ritesh Harjani
2021-03-23T15:59:30Z

On 3/19/21 7:22 AM, Shiyang Ruan wrote:
> The core logic in the two dax page fault functions is similar. So, move
> the logic into a common helper function. Also, to facilitate the
> addition of new features, such as CoW, switch-case is no longer used to
> handle different iomap types.
>
> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> ---
> fs/dax.c | 291 +++++++++++++++++++++++++++----------------------------
> 1 file changed, 145 insertions(+), 146 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 7031e4302b13..33ddad0f3091 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1053,6 +1053,66 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
> return ret;
> }
>
> +#ifdef CONFIG_FS_DAX_PMD
> +static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
> + struct iomap *iomap, void **entry)
> +{
> + struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> + unsigned long pmd_addr = vmf->address & PMD_MASK;
> + struct vm_area_struct *vma = vmf->vma;
> + struct inode *inode = mapping->host;
> + pgtable_t pgtable = NULL;
> + struct page *zero_page;
> + spinlock_t *ptl;
> + pmd_t pmd_entry;
> + pfn_t pfn;
> +
> + zero_page = mm_get_huge_zero_page(vmf->vma->vm_mm);
> +
> + if (unlikely(!zero_page))
> + goto fallback;
> +
> + pfn = page_to_pfn_t(zero_page);
> + *entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> + DAX_PMD | DAX_ZERO_PAGE, false);
> +
> + if (arch_needs_pgtable_deposit()) {
> + pgtable = pte_alloc_one(vma->vm_mm);
> + if (!pgtable)
> + return VM_FAULT_OOM;
> + }
> +
> + ptl = pmd_lock(vmf->vma->vm_mm, vmf->pmd);
> + if (!pmd_none(*(vmf->pmd))) {
> + spin_unlock(ptl);
> + goto fallback;
> + }
> +
> + if (pgtable) {
> + pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
> + mm_inc_nr_ptes(vma->vm_mm);
> + }
> + pmd_entry = mk_pmd(zero_page, vmf->vma->vm_page_prot);
> + pmd_entry = pmd_mkhuge(pmd_entry);
> + set_pmd_at(vmf->vma->vm_mm, pmd_addr, vmf->pmd, pmd_entry);
> + spin_unlock(ptl);
> + trace_dax_pmd_load_hole(inode, vmf, zero_page, *entry);
> + return VM_FAULT_NOPAGE;
> +
> +fallback:
> + if (pgtable)
> + pte_free(vma->vm_mm, pgtable);
> + trace_dax_pmd_load_hole_fallback(inode, vmf, zero_page, *entry);
> + return VM_FAULT_FALLBACK;
> +}
> +#else
> +static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
> + struct iomap *iomap, void **entry)
> +{
> + return VM_FAULT_FALLBACK;
> +}
> +#endif /* CONFIG_FS_DAX_PMD */
> +
> s64 dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap)
> {
> sector_t sector = iomap_sector(iomap, pos & PAGE_MASK);
> @@ -1289,6 +1349,61 @@ static int dax_fault_cow_page(struct vm_fault *vmf, struct iomap *iomap,
> return 0;
> }
>
> +/**
> + * dax_fault_actor - Common actor to handle pfn insertion in PTE/PMD fault.
> + * @vmf: vm fault instance
> + * @pfnp: pfn to be returned
> + * @xas: the dax mapping tree of a file
> + * @entry: an unlocked dax entry to be inserted
> + * @pmd: distinguish whether it is a pmd fault
> + * @flags: iomap flags
> + * @iomap: from iomap_begin()
> + * @srcmap: from iomap_begin(), not equal to iomap if it is a CoW
> + */
> +static vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
> + struct xa_state *xas, void *entry, bool pmd, unsigned int flags,
> + struct iomap *iomap, struct iomap *srcmap)
> +{
> + struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> + size_t size = pmd ? PMD_SIZE : PAGE_SIZE;
> + loff_t pos = (loff_t)xas->xa_offset << PAGE_SHIFT;

shouldn't we use xa_index here for pos ?
(loff_t)xas->xa_index << PAGE_SHIFT;

> + bool write = vmf->flags & FAULT_FLAG_WRITE;
> + bool sync = dax_fault_is_synchronous(flags, vmf->vma, iomap);
> + int err = 0;
> + pfn_t pfn;
> +
> + /* if we are reading UNWRITTEN and HOLE, return a hole. */
> + if (!write &&
> + (iomap->type == IOMAP_UNWRITTEN || iomap->type == IOMAP_HOLE)) {
> + if (!pmd)
> + return dax_load_hole(xas, mapping, &entry, vmf);
> + else
> + return dax_pmd_load_hole(xas, vmf, iomap, &entry);
> + }
> +
> + if (iomap->type != IOMAP_MAPPED) {
> + WARN_ON_ONCE(1);
> + return VM_FAULT_SIGBUS;
> + }

So now in case if mapping is not mapped, we always cause
VM_FAULT_SIGBUG. But earlier we were only doing WARN_ON_ONCE(1).
Can you pls help answer why the change in behavior?

> +
> + err = dax_iomap_pfn(iomap, pos, size, &pfn);
> + if (err)
> + return dax_fault_return(err);

Same case here as well. This could return SIGBUS while earlier I am not
sure why were we only returning FALLBACK?

> +
> + entry = dax_insert_entry(xas, mapping, vmf, entry, pfn, 0,
> + write && !sync);

In dax_insert_entry() we are passing 0 as flags.
We should be passing DAX_PMD/DAX_PTE no?

> +
> + if (sync)
> + return dax_fault_synchronous_pfnp(pfnp, pfn);
> +

/* handle PMD case here */
> + if (pmd)
> + return vmf_insert_pfn_pmd(vmf, pfn, write);

/* handle PTE case here */
> + if (write)
> + return vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
> + else
> + return vmf_insert_mixed(vmf->vma, vmf->address, pfn);
> +}

It is easy to miss the return from if(pmd) case while reading.
A comment like above could be helpful for code review.

> +
> static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> int *iomap_errp, const struct iomap_ops *ops)
> {
> @@ -1296,17 +1411,14 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> struct address_space *mapping = vma->vm_file->f_mapping;
> XA_STATE(xas, &mapping->i_pages, vmf->pgoff);
> struct inode *inode = mapping->host;
> - unsigned long vaddr = vmf->address;
> loff_t pos = (loff_t)vmf->pgoff << PAGE_SHIFT;
> struct iomap iomap = { .type = IOMAP_HOLE };
> struct iomap srcmap = { .type = IOMAP_HOLE };
> unsigned flags = IOMAP_FAULT;
> int error, major = 0;
> bool write = vmf->flags & FAULT_FLAG_WRITE;
> - bool sync;
> vm_fault_t ret = 0;
> void *entry;
> - pfn_t pfn;
>
> trace_dax_pte_fault(inode, vmf, ret);
> /*
> @@ -1352,8 +1464,8 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> goto unlock_entry;
> }
> if (WARN_ON_ONCE(iomap.offset + iomap.length < pos + PAGE_SIZE)) {
> - error = -EIO; /* fs corruption? */
> - goto error_finish_iomap;
> + ret = VM_FAULT_SIGBUS; /* fs corruption? */
> + goto finish_iomap;
> }
>
> if (vmf->cow_page) {
> @@ -1363,49 +1475,19 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> goto finish_iomap;
> }
>
> - sync = dax_fault_is_synchronous(flags, vma, &iomap);
> -
> - switch (iomap.type) {
> - case IOMAP_MAPPED:
> - if (iomap.flags & IOMAP_F_NEW) {
> - count_vm_event(PGMAJFAULT);
> - count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
> - major = VM_FAULT_MAJOR;
> - }
> - error = dax_iomap_pfn(&iomap, pos, PAGE_SIZE, &pfn);
> - if (error < 0)
> - goto error_finish_iomap;
> -
> - entry = dax_insert_entry(&xas, mapping, vmf, entry, pfn,
> - 0, write && !sync);
> -
> - if (sync) {
> - ret = dax_fault_synchronous_pfnp(pfnp, pfn);
> - goto finish_iomap;
> - }
> - trace_dax_insert_mapping(inode, vmf, entry);
> - if (write)
> - ret = vmf_insert_mixed_mkwrite(vma, vaddr, pfn);
> - else
> - ret = vmf_insert_mixed(vma, vaddr, pfn);
> -
> + ret = dax_fault_actor(vmf, pfnp, &xas, entry, false, flags,
> + &iomap, &srcmap);
> + if (ret == VM_FAULT_SIGBUS)
> goto finish_iomap;
> - case IOMAP_UNWRITTEN:
> - case IOMAP_HOLE:
> - if (!write) {
> - ret = dax_load_hole(&xas, mapping, &entry, vmf);
> - goto finish_iomap;
> - }
> - fallthrough;
> - default:
> - WARN_ON_ONCE(1);
> - error = -EIO;
> - break;
> +
> + /* read/write MAPPED, CoW UNWRITTEN */
> + if (iomap.flags & IOMAP_F_NEW) {
> + count_vm_event(PGMAJFAULT);
> + count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
> + major = VM_FAULT_MAJOR;
> }

It is much better if above accounting is also done in dax_fault_actor()
function itself. Then at the end of this function we need to just do
"return ret" instead of "return ret | major"

>
> - error_finish_iomap:
> - ret = dax_fault_return(error);
> - finish_iomap:
> +finish_iomap:
> if (ops->iomap_end) {
> int copied = PAGE_SIZE;
>
> @@ -1419,66 +1501,14 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> */
> ops->iomap_end(inode, pos, PAGE_SIZE, copied, flags, &iomap);
> }
> - unlock_entry:
> +unlock_entry:
> dax_unlock_entry(&xas, entry);
> - out:
> +out:
> trace_dax_pte_fault_done(inode, vmf, ret);
> return ret | major;
> }

-ritesh