On Fri, Dec 31, 2010 at 10:16:35PM +1100, Nick Piggin wrote: > On Fri, Dec 31, 2010 at 10:00 PM, Joel Becker <jlbec@xxxxxxxxxxxx> wrote: > > On Fri, Dec 31, 2010 at 08:31:41PM +1100, Nick Piggin wrote: > >> On Fri, Dec 31, 2010 at 10:29 AM, Joel Becker <Joel.Becker@xxxxxxxxxx> wrote: > > >> The better way to do this would be to just return VM_FAULT_NOPAGE > >> in any case you need the VM to retry the fault. When you reach the > >> business end of your handler, you want to hold the page locked, after > >> you verify it is correct, and return that to the fault handler. > > > > This is going to be hard. Our write_end() assumes it must > > unlock the pages (which is normal behavior for write(2)), but in the > > page_mkwrite() case we need to avoid the unlock to follow your > > recommendation (we use our write_begin/write_end pair to trigger any > > allocation or zeroing needed before the page is writable). > > Yes that would be the best option. It is possible to use the unlocked > return, but that still gives possibility of races in some cases. Consider Yes, I am aware of that. > Basically it would be nice for all filesystems to move to this convention > so we can remove the old cruft. I can appreciate that. And you've just answered the "Do you want us to get there, or are minor faults in the old-0-style OK?" question. > > The find_or_create_page() is deep at the meat of the function, > > not the cursory check at the top. The idea is that at this point, > > find_or_create_page() will return a locked page that must, by > > definition, be part of the correct mapping. > > But you must still handle failures there, because find_or_create_page > may return -ENOMEM. So just lock the page, recheck the mapping > there, and then do exactly the same error handling. Of course it can error, but error is different than clean restart. Though cleanup should be the same; I'm looking at our code trying to convince myself that this is so ;-) Btw, -ENOMEM is that OOM fault error, right? ;-) > > If the VM is rechecking the pte after we return from > > page_mkwrite(), won't it see any new page created? > > But the point of page_mkwrite is a dirty notifier for the fs. If this new > different page was installed due to say truncate then a read-only > fault, the filesystem would miss the notification. So it just does the > simple thing and retries the whole sequence (if needed). Fair enough, even if we caused the new page and thus are already notified ;-) Essentially, you would really like us (and ext4, and gfs2, etc) to start returning VM_FAULT_NOPAGE for any place we aren't sure what happened to our page, VM_FAULT_OOM when we get an -ENOMEM, and VM_FAULT_LOCKED for all successful operations. That the long and short of it? Thank you for helping me understand your plan for this code. Btw, what not use VM_FAULT_RETRY for "I'd like you to retry"? Especially since the comment for NOPAGE doesn't read anything like its actual behavior. Joel -- "Friends may come and go, but enemies accumulate." - Thomas Jones Joel Becker Senior Development Manager Oracle E-mail: joel.becker@xxxxxxxxxx Phone: (650) 506-8127 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html