On Tue, Jun 28, 2022 at 05:31:43PM -0700, John Hubbard wrote: > On 6/28/22 15:33, Peter Xu wrote: > > > The key point is the connection between "locked" and killable. If the comment > > > explained why "locked" means "killable", that would help clear this up. The > > > NOWAIT sentence is also confusing to me, and adding "mostly NOWAIT" does not > > > clear it up either... :) > > > > Sorry to have a comment that makes it feels confusing. I tried to > > explicitly put the comment to be after setting FAULT_FLAG_KILLABLE but > > obviously I didn't do my job well.. > > > > Maybe that NOWAIT thing adds more complexity but not even necessary. > > > > Would below one more acceptable? > > > > /* > > * We'll only be able to respond to signals when "locked != > > * NULL". When with it, we'll always respond to SIGKILL > > * (as implied by FAULT_FLAG_KILLABLE above), and we'll > > * respond to non-fatal signals only if the GUP user has > > * specified FOLL_INTERRUPTIBLE. > > */ > > > It looks like part of this comment is trying to document a pre-existing > concept, which is that faultin_page() only ever sets FAULT_FLAG_KILLABLE > if locked != NULL. I'd say that's not what I wanted to comment.. I wanted to express that INTERRUPTIBLE should rely on KILLABLE, that's also why I put the comment to be after KILLABLE, not before. IMHO it makes sense already to have "interruptible" only if "killable", no matter what's the pre-requisite for KILLABLE (in this case it's having "locked" being non-null). > The problem I am (personally) having is that I don't yet understand why > or how those are connected: what is it about having locked non-NULL that > means the process is killable? (Can you explain why that is?) Firstly RETRY_KILLABLE relies on ALLOW_RETRY, because if we don't allow retry at all it means we'll never wait in handle_mm_fault() anyway, then no need to worry on being interrupted by any kind of signal (fatal or not). Then if we allow retry, we need some way to know "whether mmap_sem is released or not" during the process for the caller (because the caller cannot see VM_FAULT_RETRY). That's why we added "locked" parameter, so that we can set *locked=false to tell the caller we have released mmap_sem. I think that's why we have "locked" defined as "we allow this page fault request to retry and wait, during wait we can always allow fatal signals". I think that's defined throughout the gup call interfaces too, and faultin_page() is the last step to talk to handle_mm_fault(). To make this whole picture complete, NOWAIT is another thing that relies on ALLOW_RETRY but just to tell "oh please never release the mmap_sem at all". For example, when we want to make sure no vma will be released after faultin_page() returned. > > If that were clear, I think I could suggest a good comment wording. IMHO it's a little bit weird to explain "locked" here, especially after KILLABLE is set, that's why I didn't try to mention "locked" in my 2nd attempt. There are some comments for "locked" above the definition of faultin_page(), I think that'll be a nicer place to enrich explanations for "locked", and it seems even more suitable as a separate patch? Thanks, -- Peter Xu
