Re: [PATCH v4 10/12] dax: add struct iomap based DAX PMD support

Ross Zwisler
2016-10-04T15:39:53Z

On Tue, Oct 04, 2016 at 07:55:57AM +0200, Jan Kara wrote:
> On Mon 03-10-16 15:05:57, Ross Zwisler wrote:
> > > > @@ -623,22 +672,30 @@ static void *dax_insert_mapping_entry(struct address_space *mapping,
> > > > error = radix_tree_preload(vmf->gfp_mask & ~__GFP_HIGHMEM);
> > > > if (error)
> > > > return ERR_PTR(error);
> > > > + } else if ((unsigned long)entry & RADIX_DAX_HZP && !hzp) {
> > > > + /* replacing huge zero page with PMD block mapping */
> > > > + unmap_mapping_range(mapping,
> > > > + (vmf->pgoff << PAGE_SHIFT) & PMD_MASK, PMD_SIZE, 0);
> > > > }
> > > >
> > > > spin_lock_irq(&mapping->tree_lock);
> > > > - new_entry = (void *)((unsigned long)RADIX_DAX_ENTRY(sector, false) |
> > > > - RADIX_DAX_ENTRY_LOCK);
> > > > + if (hzp)
> > > > + new_entry = RADIX_DAX_HZP_ENTRY();
> > > > + else
> > > > + new_entry = RADIX_DAX_ENTRY(sector, new_type);
> > > > +
> > > > if (hole_fill) {
> > > > __delete_from_page_cache(entry, NULL);
> > > > /* Drop pagecache reference */
> > > > put_page(entry);
> > > > - error = radix_tree_insert(page_tree, index, new_entry);
> > > > + error = __radix_tree_insert(page_tree, index,
> > > > + RADIX_DAX_ORDER(new_type), new_entry);
> > > > if (error) {
> > > > new_entry = ERR_PTR(error);
> > > > goto unlock;
> > > > }
> > > > mapping->nrexceptional++;
> > > > - } else {
> > > > + } else if ((unsigned long)entry & (RADIX_DAX_HZP|RADIX_DAX_EMPTY)) {
> > > > void **slot;
> > > > void *ret;
> > >
> > > Hum, I somewhat dislike how PTE and PMD paths differ here. But it's OK for
> > > now I guess. Long term we might be better off to do away with zero pages
> > > for PTEs as well and use exceptional entry and a single zero page like you
> > > do for PMD. Because the special cases these zero pages cause are a
> > > headache.
> >
> > I've been thinking about this as well, and I do think we'd be better off with
> > a single zero page for PTEs, as we have with PMDs. It'd reduce the special
> > casing in the DAX code, and it'd also ensure that we don't waste a bunch of
> > time and memory creating read-only zero pages to service reads from holes.
> >
> > I'll look into adding this for v5.
>
> Well, this would clash with the dirty bit cleaning series I have. So I'd
> prefer to put this on a todo list and address it once existing series are
> integrated...

Sure, that works.

> > > > + if (error)
> > > > + goto fallback;
> > > > + if (iomap.offset + iomap.length < pos + PMD_SIZE)
> > > > + goto fallback;
> > > > +
> > > > + vmf.pgoff = pgoff;
> > > > + vmf.flags = flags;
> > > > + vmf.gfp_mask = mapping_gfp_mask(mapping) | __GFP_FS | __GFP_IO;
> > >
> > > I don't think you want __GFP_FS here - we have already gone through the
> > > filesystem's pmd_fault() handler which called dax_iomap_pmd_fault() and
> > > thus we hold various fs locks, freeze protection, ...
> >
> > I copied this from __get_fault_gfp_mask() in mm/memory.c. That function is
> > used by do_page_mkwrite() and __do_fault(), and we eventually get this
> > vmf->gfp_mask in the PTE fault code. With the code as it is we get the same
> > vmf->gfp_mask in both dax_iomap_fault() and dax_iomap_pmd_fault(). It seems
> > like they should remain consistent - is it wrong to have __GFP_FS in
> > dax_iomap_fault()?
>
> The gfp_mask that propagates from __do_fault() or do_page_mkwrite() is fine
> because at that point it is correct. But once we grab filesystem locks
> which are not reclaim safe, we should update vmf->gfp_mask we pass further
> down into DAX code to not contain __GFP_FS (that's a bug we apparently have
> there). And inside DAX code, we definitely are not generally safe to add
> __GFP_FS to mapping_gfp_mask(). Maybe we'd be better off propagating struct
> vm_fault into this function, using passed gfp_mask there and make sure
> callers update gfp_mask as appropriate.

Yep, that makes sense to me. In reviewing your set it also occurred to me that
we might want to stick a struct vm_area_struct *vma pointer in the vmf, since
you always need a vma when you are using a vmf, but we pass them as a pair
everywhere.