On Thu, Jun 11, 2015 at 8:49 AM, Ingo Molnar <mi...@kernel.org> wrote: > > * Andy Lutomirski <l...@amacapital.net> wrote: > >> On Thu, Jun 11, 2015 at 7:07 AM, Ingo Molnar <mi...@kernel.org> wrote: >> > The vmalloc() code uses vmalloc_sync_all() to synchronize changes to >> > the global reference kernel PGD to task PGDs. >> >> Does it? AFAICS the only caller is register_die_notifier, and it's >> not really clear to me why that exists. > > Doh, indeed, got confused in that changelog - we are filling it in > opportunistically via vmalloc_fault(). > >> At some point I'd love to remove lazy kernel PGD sync from the kernel >> entirely >> (or at least from x86) and just do it when we switch mms. Now that you're >> removing all code that deletes kernel PGD entries, I think all we'd need to >> do >> is to add a per-PGD or per-mm count of the number of kernel entries populated >> and to fix it up when we switch to an mm with fewer entries populated than >> init_mm. > > That would add a (cheap but nonzero) runtime check to every context switch. > It's a > relative slow path, but in comparison vmalloc() is an even slower slowpath, > so why > not do it there and just do synchronous updates and remove the vmalloc faults > altogether? > > Also, on 64-bit it should not matter much: there the only change is the once > in a > blue moon case where we allocate a new pgd for a 512 GB block of address space > that a single pgd entry covers. > > I'd hate to add a check to every context switch, no matter how cheap, just > for a > case that essentially never triggers... > > So how about this solution instead: > > - we add a generation counter to sync_global_pgds() so that it can detect > when > the number of pgds populated in init_mm changes. > > - we change vmalloc() to call sync_global_pgds(): this will be very cheap in > the > overwhelming majority of cases. > > - we eliminate vmalloc_fault(), on 64-bit at least. Yay! :-)
Should be good. The only real down side is that the update becomes a walk over all tasks or over all mms, whereas if we checked on context switches, then the update is just a single PGD entry copy. That being said, the updates should be *really* rare. I think it's completely impossible for it to happen more than 255 times per boot. So I like your solution. Then I can stop wondering what happens when we manage to take a vmalloc fault early in entry_SYSCALL_64 or somewhere in the page_fault prologue. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
