From: Jens Axboe axboe@kernel.dk
[ Upstream commit 8fb0f47a9d7acf620d0fd97831b69da9bc5e22ed ]
In an ideal world, when someone is passed an iov_iter and returns X bytes, then X bytes would have been consumed/advanced from the iov_iter. But we have use cases that always consume the entire iterator, a few examples of that are iomap and bdev O_DIRECT. This means we cannot rely on the state of the iov_iter once we've called ->read_iter() or ->write_iter().
This would be easier if we didn't always have to deal with truncate of the iov_iter, as rewinding would be trivial without that. We recently added a commit to track the truncate state, but that grew the iov_iter by 8 bytes and wasn't the best solution.
Implement a helper to save enough of the iov_iter state to sanely restore it after we've called the read/write iterator helpers. This currently only works for IOVEC/BVEC/KVEC as that's all we need, support for other iterator types are left as an exercise for the reader.
Link: https://lore.kernel.org/linux-fsdevel/CAHk-=wiacKV4Gh-MYjteU0LwNBSGpWrK-Ov25... Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/uio.h | 15 +++++++++++++++ lib/iov_iter.c | 52 +++++++++++++++++++++++++++++++++------------------- 2 files changed, 48 insertions(+), 19 deletions(-)
--- a/include/linux/uio.h +++ b/include/linux/uio.h @@ -26,6 +26,12 @@ enum iter_type { ITER_DISCARD = 64, };
+struct iov_iter_state { + size_t iov_offset; + size_t count; + unsigned long nr_segs; +}; + struct iov_iter { /* * Bit 0 is the read/write bit, set if we're writing. @@ -55,6 +61,14 @@ static inline enum iter_type iov_iter_ty return i->type & ~(READ | WRITE); }
+static inline void iov_iter_save_state(struct iov_iter *iter, + struct iov_iter_state *state) +{ + state->iov_offset = iter->iov_offset; + state->count = iter->count; + state->nr_segs = iter->nr_segs; +} + static inline bool iter_is_iovec(const struct iov_iter *i) { return iov_iter_type(i) == ITER_IOVEC; @@ -226,6 +240,7 @@ ssize_t iov_iter_get_pages(struct iov_it ssize_t iov_iter_get_pages_alloc(struct iov_iter *i, struct page ***pages, size_t maxsize, size_t *start); int iov_iter_npages(const struct iov_iter *i, int maxpages); +void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state);
const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags);
--- a/lib/iov_iter.c +++ b/lib/iov_iter.c @@ -1836,24 +1836,38 @@ int import_single_range(int rw, void __u } EXPORT_SYMBOL(import_single_range);
-int iov_iter_for_each_range(struct iov_iter *i, size_t bytes, - int (*f)(struct kvec *vec, void *context), - void *context) +/** + * iov_iter_restore() - Restore a &struct iov_iter to the same state as when + * iov_iter_save_state() was called. + * + * @i: &struct iov_iter to restore + * @state: state to restore from + * + * Used after iov_iter_save_state() to bring restore @i, if operations may + * have advanced it. + * + * Note: only works on ITER_IOVEC, ITER_BVEC, and ITER_KVEC + */ +void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state) { - struct kvec w; - int err = -EINVAL; - if (!bytes) - return 0; - - iterate_all_kinds(i, bytes, v, -EINVAL, ({ - w.iov_base = kmap(v.bv_page) + v.bv_offset; - w.iov_len = v.bv_len; - err = f(&w, context); - kunmap(v.bv_page); - err;}), ({ - w = v; - err = f(&w, context);}) - ) - return err; + if (WARN_ON_ONCE(!iov_iter_is_bvec(i) && !iter_is_iovec(i)) && + !iov_iter_is_kvec(i)) + return; + i->iov_offset = state->iov_offset; + i->count = state->count; + /* + * For the *vec iters, nr_segs + iov is constant - if we increment + * the vec, then we also decrement the nr_segs count. Hence we don't + * need to track both of these, just one is enough and we can deduct + * the other from that. ITER_KVEC and ITER_IOVEC are the same struct + * size, so we can just increment the iov pointer as they are unionzed. + * ITER_BVEC _may_ be the same size on some archs, but on others it is + * not. Be safe and handle it separately. + */ + BUILD_BUG_ON(sizeof(struct iovec) != sizeof(struct kvec)); + if (iov_iter_is_bvec(i)) + i->bvec -= state->nr_segs - i->nr_segs; + else + i->iov -= state->nr_segs - i->nr_segs; + i->nr_segs = state->nr_segs; } -EXPORT_SYMBOL(iov_iter_for_each_range);