Infiniband use of get_user_pages()

[Jan Kara <jack@suse.cz>]

  Hello,

  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?

  Also that seems to be the only real reason why mmap_sem has to be grabbed
in exclusive mode, am I right?

  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?

  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?


								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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