On Fri, Dec 31, 2010 at 10:39 PM, Joel Becker <Joel.Becker@xxxxxxxxxx> wrote: > 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. It is OK in that it will work. It is not preferred, if you are reworking pleaase use the new style. >> > 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 ;-) Cleanup is exactly the same, yes, because the only difference in different error codes is what the VM does with them. > Btw, -ENOMEM is that OOM > fault error, right? ;-) 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 ;-) Well, you would be notified via ->fault, but not not writable fault and no page_mkwrite. > 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. Exactly. > 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. It was designed first to replace that old ->nopfn handler, so it does need comments updating. RETRY doesn't cover the ->nopfn usage, but NOPAGE does cover the retry usage, I think, so names are OK. -- 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
