On Mon 23-01-23 16:31:32, Matthew Wilcox wrote: > On Fri, Jan 20, 2023 at 05:55:48PM +0000, David Howells wrote: > > (3) Make the bio struct carry a pair of flags to indicate the cleanup > > mode. BIO_NO_PAGE_REF is replaced with BIO_PAGE_REFFED (equivalent to > > FOLL_GET) and BIO_PAGE_PINNED (equivalent to BIO_PAGE_PINNED) is > > added. > > I think there's a simpler solution than all of this. > > As I understand the fundamental problem here, the question is > when to copy a page on fork. We have the optimisation of COW, but > O_DIRECT/RDMA/... breaks it. So all this page pinning is to indicate > to the fork code "You can't do COW to this page". > > Why do we want to track that information on a per-page basis? Wouldn't it > be easier to have a VM_NOCOW flag in vma->vm_flags? Set it the first > time somebody does an O_DIRECT read or RDMA pin. That's it. Pages in > that VMA will now never be COWed, regardless of their refcount/mapcount. > And the whole "did we pin or get this page" problem goes away. Along > with folio->pincount. Well, but anon COW code is not the only (planned) consumer of the pincount. Filesystems also need to know whether a (shared pagecache) page is pinned and can thus be modified behind their backs. And for that VMA tracking isn't really an option. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
