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SubjectRe: [PATCH v7 07/22] Replace the XIP page fault handler with the DAX page fault handler
On Wed, Apr 09, 2014 at 12:05:25AM +0200, Jan Kara wrote:
> > + if (!page)
> > + return VM_FAULT_OOM;
> > + size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > + if (vmf->pgoff >= size) {
> Maybe comment here that we have to recheck i_size so that we don't create
> pages in the area truncate_pagecache() has already evicted.

Done.

> > + dax_get_addr(inode, bh, &vfrom); /* XXX: error handling */
> The error handling here is missing as the comment suggests :)

Added.

> > + if (buffer_unwritten(&bh) || buffer_new(&bh))
> > + dax_clear_blocks(inode, bh.b_blocknr, bh.b_size);
> Where is dax_clear_blocks() defined?

Er ... patch 11. I'll reorder the patches ;-)

> > +
> > + error = dax_get_pfn(inode, &bh, &pfn);
> > + if (error > 0)
> > + error = vm_insert_mixed(vma, vaddr, pfn);
> When there's a hole (thus page != NULL) and we are called from
> dax_mkwrite(), this will always return EBUSY, correct?

Erm ... it will return -EBUSY if this was the task that previously
faulted on it. Drat. See below.

> > + mutex_unlock(&mapping->i_mmap_mutex);
> > +
> > + if (page) {
> > + delete_from_page_cache(page);
> > + unmap_mapping_range(mapping, vmf->pgoff << PAGE_SHIFT,
> > + PAGE_CACHE_SIZE, 0);
> Here we unmap the PTE pointing to the hole page but then we'll have to
> retry the fault again to fill in the pfn we've got? This seems wrong. I'd
> say we want to remap the PTE from the hole page to a pfn we've got while
> holding i_mmap_mutex. remap_pfn_range() almost does what you need, except
> that you also need that to work for normal pages. So you might need to
> create a new helper in mm layer for that.

I think it's easier than that. How does this look?

@@ -390,9 +389,8 @@ static int do_dax_fault(struct vm_area_struct *vma, struct v
dax_clear_blocks(inode, bh.b_blocknr, bh.b_size);

error = dax_get_pfn(&bh, &pfn, blkbits);
- if (error > 0)
- error = vm_insert_mixed(vma, vaddr, pfn);
- mutex_unlock(&mapping->i_mmap_mutex);
+ if (error <= 0)
+ goto unlock;

if (page) {
delete_from_page_cache(page);
@@ -402,6 +400,9 @@ static int do_dax_fault(struct vm_area_struct *vma, struct v
page_cache_release(page);
}

+ error = vm_insert_mixed(vma, vaddr, pfn);
+ mutex_unlock(&mapping->i_mmap_mutex);
+
if (error == -ENOMEM)
return VM_FAULT_OOM;
/* -EBUSY is fine, somebody else faulted on the same PTE */
@@ -409,6 +410,8 @@ static int do_dax_fault(struct vm_area_struct *vma, struct v
BUG_ON(error);
return VM_FAULT_NOPAGE | major;

+ unlock:
+ mutex_unlock(&mapping->i_mmap_mutex);
sigbus:
if (page) {
unlock_page(page);

> > +int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
> > + get_block_t get_block)
> > +{
> > + int result;
> > + struct super_block *sb = file_inode(vma->vm_file)->i_sb;
> > +
> > + sb_start_pagefault(sb);
> You don't need any filesystem freeze protection for the fault handler
> since that's not going to modify the filesystem.

Err ... we might allocate a block as a result of doing a write to a hole.
Or does that not count as 'modifying the filesystem' in this context?

> > + file_update_time(vma->vm_file);
> Why do you update m/ctime? We are only reading the file...

... except that it might be a write fault. I think we modify the file
iff we return VM_FAULT_MAJOR from do_dax_fault(). So I'd be open to
something like this:

sb_start_pagefault(sb);
result = do_dax_fault(vma, vmf, get_block);
if (result & VM_FAULT_MAJOR)
file_update_time(vma->vm_file);
sb_end_pagefault(sb);
Would that work better for you?

> > @@ -70,7 +101,7 @@ const struct file_operations ext2_file_operations = {
> > #ifdef CONFIG_COMPAT
> > .compat_ioctl = ext2_compat_ioctl,
> > #endif
> > - .mmap = generic_file_mmap,
> > + .mmap = ext2_file_mmap,
> So what's the point of ext2_file_operations ever handling IS_DAX()
> inodes? Actually ext2_file_operations and ext2_xip_file_operations seem to
> be the same after this patch so either you drop ext2_xip_file_operations
> (I'm for this) or you can leave generic_file_mmap here and assume
> ext2_file_mmap is always called for IS_DAX() inodes.

The goal is to get them the same. At this point, the only sticky point is:

.splice_read = generic_file_splice_read,
.splice_write = generic_file_splice_write,

And splice is pretty damn sticky for DAX.



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