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

[ruansy.fnst@xxxxxxxxxxx]

> -----Original Message-----
> From: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
> 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@xxxxxxxxxxx>
> > ---
> >   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