Messages in this thread | | | Date | Thu, 25 Jun 2015 15:57:29 -0700 | From | Mark Hairgrove <> | Subject | Re: [PATCH 06/36] HMM: add HMM page table v2. |
| |
On Thu, 21 May 2015, j.glisse@gmail.com wrote:
> From: Jérôme Glisse <jglisse@redhat.com> > > [...] > + > +void hmm_pt_iter_init(struct hmm_pt_iter *iter); > +void hmm_pt_iter_fini(struct hmm_pt_iter *iter, struct hmm_pt *pt); > +unsigned long hmm_pt_iter_next(struct hmm_pt_iter *iter, > + struct hmm_pt *pt, > + unsigned long addr, > + unsigned long end); > +dma_addr_t *hmm_pt_iter_update(struct hmm_pt_iter *iter, > + struct hmm_pt *pt, > + unsigned long addr); > +dma_addr_t *hmm_pt_iter_fault(struct hmm_pt_iter *iter, > + struct hmm_pt *pt, > + unsigned long addr);
I've got a few more thoughts on hmm_pt_iter after looking at some of the later patches. I think I've convinced myself that this patch functionally works as-is, but I've got some suggestions and questions about the design.
Right now there are these three major functions:
1) hmm_pt_iter_update(addr) - Returns the hmm_pte * for addr, or NULL if none exists.
2) hmm_pt_iter_fault(addr) - Returns the hmm_pte * for addr, allocating a new one if none exists.
3) hmm_pt_iter_next(addr, end) - Returns the next possibly-valid address. The caller must use hmm_pt_iter_update to check if there really is an hmm_pte there.
In my view, there are two sources of confusion here: - Naming. "update" shares a name with the HMM mirror callback, and it also implies that the page tables are "updated" as a result of the call. "fault" likewise implies that the function handles a fault in some way. Neither of these implications are true.
- hmm_pt_iter_next and hmm_pt_iter_update have some overlapping functionality when compared to traditional iterators, requiring the callers to all do this sort of thing:
hmm_pte = hmm_pt_iter_update(&iter, &mirror->pt, addr); if (!hmm_pte) { addr = hmm_pt_iter_next(&iter, &mirror->pt, addr, event->end); continue; }
Wouldn't it be more efficient and simpler to have _next do all the iteration internally so it always returns the next valid entry? Then you could combine _update and _next into a single function, something along these lines (which also addresses the naming concern):
void hmm_pt_iter_init(iter, pt, start, end); unsigned long hmm_pt_iter_next(iter, hmm_pte *); unsigned long hmm_pt_iter_next_alloc(iter, hmm_pte *);
hmm_pt_iter_next would return the address and ptep of the next valid entry, taking the place of the existing _update and _next functions. hmm_pt_iter_next_alloc takes the place of _fault.
Also, since the _next functions don't take in an address, the iterator doesn't have to handle the input addr being different from iter->cur.
The logical extent of this is a callback approach like mm_walk. That would be nice because the caller wouldn't have to worry about making the _init and _fini calls. I assume you didn't go with this approach because sometimes you need to iterate over hmm_pt while doing an mm_walk itself, and you didn't want the overhead of nesting those?
Finally, another minor thing I just noticed: shouldn't hmm_pt.h include <linux/bitops.h> since it uses all of the clear/set/test bit APIs?
| |