iov_iter: add helper to save iov_iter state
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-Ov25HdqB1ewinrFPg@mail.gmail.com/ Signed-off-by: Jens Axboe <axboe@kernel.dk>
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@ -27,6 +27,12 @@ enum iter_type {
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ITER_DISCARD,
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};
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struct iov_iter_state {
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size_t iov_offset;
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size_t count;
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unsigned long nr_segs;
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};
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struct iov_iter {
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u8 iter_type;
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bool data_source;
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@ -55,6 +61,14 @@ static inline enum iter_type iov_iter_type(const struct iov_iter *i)
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return i->iter_type;
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}
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static inline void iov_iter_save_state(struct iov_iter *iter,
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struct iov_iter_state *state)
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{
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state->iov_offset = iter->iov_offset;
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state->count = iter->count;
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state->nr_segs = iter->nr_segs;
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}
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static inline bool iter_is_iovec(const struct iov_iter *i)
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{
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return iov_iter_type(i) == ITER_IOVEC;
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@ -233,6 +247,7 @@ ssize_t iov_iter_get_pages(struct iov_iter *i, struct page **pages,
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ssize_t iov_iter_get_pages_alloc(struct iov_iter *i, struct page ***pages,
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size_t maxsize, size_t *start);
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int iov_iter_npages(const struct iov_iter *i, int maxpages);
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void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state);
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const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags);
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@ -1972,3 +1972,39 @@ int import_single_range(int rw, void __user *buf, size_t len,
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return 0;
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}
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EXPORT_SYMBOL(import_single_range);
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/**
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* iov_iter_restore() - Restore a &struct iov_iter to the same state as when
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* iov_iter_save_state() was called.
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*
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* @i: &struct iov_iter to restore
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* @state: state to restore from
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*
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* Used after iov_iter_save_state() to bring restore @i, if operations may
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* have advanced it.
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*
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* Note: only works on ITER_IOVEC, ITER_BVEC, and ITER_KVEC
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*/
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void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state)
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{
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if (WARN_ON_ONCE(!iov_iter_is_bvec(i) && !iter_is_iovec(i)) &&
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!iov_iter_is_kvec(i))
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return;
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i->iov_offset = state->iov_offset;
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i->count = state->count;
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/*
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* For the *vec iters, nr_segs + iov is constant - if we increment
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* the vec, then we also decrement the nr_segs count. Hence we don't
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* need to track both of these, just one is enough and we can deduct
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* the other from that. ITER_KVEC and ITER_IOVEC are the same struct
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* size, so we can just increment the iov pointer as they are unionzed.
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* ITER_BVEC _may_ be the same size on some archs, but on others it is
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* not. Be safe and handle it separately.
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*/
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BUILD_BUG_ON(sizeof(struct iovec) != sizeof(struct kvec));
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if (iov_iter_is_bvec(i))
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i->bvec -= state->nr_segs - i->nr_segs;
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else
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i->iov -= state->nr_segs - i->nr_segs;
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i->nr_segs = state->nr_segs;
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}
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