On Tue, Oct 7, 2014 at 8:52 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > On Tue, Oct 07, 2014 at 04:19:13PM +0200, Andrea Arcangeli wrote: >> mremap like interface, or file+commands protocol interface. I tend to >> like mremap more, that's why I opted for a remap_anon_pages syscall >> kept orthogonal to the userfaultfd functionality (remap_anon_pages >> could be also used standalone as an accelerated mremap in some >> circumstances) but nothing prevents to just embed the same mechanism > > Sorry for the self followup, but something else comes to mind to > elaborate this further. > > In term of interfaces, the most efficient I could think of to minimize > the enter/exit kernel, would be to append the "source address" of the > data received from the network transport, to the userfaultfd_write() > command (by appending 8 bytes to the wakeup command). Said that, > mixing the mechanism to be notified about userfaults with the > mechanism to resolve an userfault to me looks a complication. I kind > of liked to keep the userfaultfd protocol is very simple and doing > just its thing. The userfaultfd doesn't need to know how the userfault > was resolved, even mremap would work theoretically (until we run out > of vmas). I thought it was simpler to keep it that way. However if we > want to resolve the fault with a "write()" syscall this may be the > most efficient way to do it, as we're already doing a write() into the > pseudofd to wakeup the page fault that contains the destination > address, I just need to append the source address to the wakeup command. > > I probably grossly overestimated the benefits of resolving the > userfault with a zerocopy page move, sorry. So if we entirely drop the > zerocopy behavior and the TLB flush of the old page like you > suggested, the way to keep the userfaultfd mechanism decoupled from > the userfault resolution mechanism would be to implement an > atomic-copy syscall. That would work for SIGBUS userfaults too without > requiring a pseudofd then. It would be enough then to call > mcopy_atomic(userfault_addr,tmp_addr,len) with the only constraints > that len must be a multiple of PAGE_SIZE. Of course mcopy_atomic > wouldn't page fault or call GUP into the destination address (it can't > otherwise the in-flight partial copy would be visible to the process, > breaking the atomicity of the copy), but it would fill in the > pte/trans_huge_pmd with the same strict behavior that remap_anon_pages > currently has (in turn it would by design bypass the VM_USERFAULT > check and be ideal for resolving userfaults). At the risk of asking a possibly useless question, would it make sense to splice data into a userfaultfd? --Andy > > mcopy_atomic could then be also extended to tmpfs and it would work > without requiring the source page to be a tmpfs page too without > having to convert page types on the fly. > > If I add mcopy_atomic, the patch in subject (10/17) can be dropped of > course so it'd be even less intrusive than the current > remap_anon_pages and it would require zero TLB flush during its > runtime (it would just require an atomic copy). > > So should I try to embed a mcopy_atomic inside userfault_write or can > I expose it to userland as a standalone new syscall? Or should I do > something different? Comments? > > Thanks, > Andrea -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
