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

ruansy.fnst
2021-03-31T03:58:47Z

> -----Original Message-----
> From: Ritesh Harjani <riteshh@linux.ibm.com>
> Sent: Tuesday, March 23, 2021 11:48 PM
> Subject: Re: [PATCH v3 02/10] fsdax: Factor helper: dax_fault_actor()
>
>
>
> 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;
Yes.
>
> > + 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?
>

The behavior in PTE fault was always check the error code by dax_fault_return(error) after WARN_ON_ONCE(1). So, I moved the dax_fault_return() into dax_fault_actor(). But I just found that, in PMD fault, it didn't do this check. So, I think I should move dax_fault_return() outside the dax_fault_actor() to keep the previous logic.

>
>
>
> > +
> > + 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?
>
Yes. Thanks for pointing out.
>
> > +
> > + 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?
>
My mistake.
>
> > +
> > + 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"
>
Yes.

--
Thanks,
Ruan Shiyang.
>
> >
> > - 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