On Wed 24-04-13 15:25:25, Roland Dreier wrote: > On Wed, Apr 24, 2013 at 8:38 AM, Jan Kara <jack@xxxxxxx> wrote: > > when checking users of get_user_pages() (I'm doing some cleanups in that > > area to fix filesystem's issues with mmap_sem locking) I've noticed that > > infiniband drivers add number of pages obtained from get_user_pages() to > > mm->pinned_vm counter. Although this makes some sence, it doesn't match > > with any other user of get_user_pages() (e.g. direct IO) so has infiniband > > some special reason why it does so? > > Direct IO mappings are in some sense ephemeral -- they only need to > last while the IO is in flight. In contrast the IB memory pinning is > controlled by (possibly unprivileged) userspace and might last the > whole lifetime of a long-lived application. So we want some > accounting and resource control. I see, thanks for explanation. > > Also that seems to be the only real reason why mmap_sem has to be grabbed > > in exclusive mode, am I right? > > Most likely that is true. > > > Another suspicious thing (at least in drivers/infiniband/core/umem.c: > > ib_umem_get()) is that arguments of get_user_pages() are like: > > ret = get_user_pages(current, current->mm, cur_base, > > min_t(unsigned long, npages, > > PAGE_SIZE / sizeof (struct page *)), > > 1, !umem->writable, page_list, vma_list); > > So we always have write argument set to 1 and force argument is set to > > !umem->writable. Is that really intentional? My naive guess would be that > > arguments should be switched... Although even in that case I fail to see > > why 'force' argument should be set. Can someone please explain? > > This confused even me recently. We had a long discussion (read the > whole thread starting here: https://lkml.org/lkml/2012/1/26/7) but in > short the current parameters seem to be needed to trigger COW even > when the kernel/hardware want to read the memory, to avoid problems > where we get stale data if userspace triggers COW. Thanks for the pointer. That was an interesting read :). > I think I better add a comment explaining this. > > > Finally (and here I may show my ignorance ;), I'd like to ask whether > > there's any reason why ib_umem_get() checks for is_vm_hugetlb_page() and > > not just whether a page is a huge page? > > I'm not sure of the history here. How would one check directly if a > page is a huge page? PageHuge(page) should do it (see mm/hugetlb.c). > get_user_pages() actually goes to some trouble to return all small pages, > even when it has to split a single huge page into many entries in the > page array. (Which is actually a bit unfortunate for our use here) Does it? As far as I'm checking get_user_pages() and the fault path I don't see where it would be happening... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>