On Sat, 30 Jun 2018 06:39:44 +0800 Yang Shi <yang....@linux.alibaba.com> wrote:
And... > diff --git a/mm/mmap.c b/mm/mmap.c > index 87dcf83..d61e08b 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -2763,6 +2763,128 @@ static int munmap_lookup_vma(struct mm_struct *mm, > struct vm_area_struct **vma, > return 1; > } > > +/* Consider PUD size or 1GB mapping as large mapping */ > +#ifdef HPAGE_PUD_SIZE > +#define LARGE_MAP_THRESH HPAGE_PUD_SIZE > +#else > +#define LARGE_MAP_THRESH (1 * 1024 * 1024 * 1024) > +#endif So this assumes that 32-bit machines cannot have 1GB mappings (fair enough) and this is the sole means by which we avoid falling into the "len >= LARGE_MAP_THRESH" codepath, which will behave very badly, at least because for such machines, VM_DEAD is zero. This is rather ugly and fragile. And, I guess, explains why we can't give all mappings this treatment: 32-bit machines can't do it. And we're adding a bunch of code to 32-bit kernels which will never be executed. I'm thinking it would be better to be much more explicit with "#ifdef CONFIG_64BIT" in this code, rather than relying upon the above magic. But I tend to think that the fact that we haven't solved anything on locked vmas or on uprobed mappings is a shostopper for the whole approach :(
