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1 2 Immutable biovecs and biovec iterators: 3 ======================================= 4 5 Kent Overstreet <firstname.lastname@example.org> 6 7 As of 3.13, biovecs should never be modified after a bio has been submitted. 8 Instead, we have a new struct bvec_iter which represents a range of a biovec - 9 the iterator will be modified as the bio is completed, not the biovec. 10 11 More specifically, old code that needed to partially complete a bio would 12 update bi_sector and bi_size, and advance bi_idx to the next biovec. If it 13 ended up partway through a biovec, it would increment bv_offset and decrement 14 bv_len by the number of bytes completed in that biovec. 15 16 In the new scheme of things, everything that must be mutated in order to 17 partially complete a bio is segregated into struct bvec_iter: bi_sector, 18 bi_size and bi_idx have been moved there; and instead of modifying bv_offset 19 and bv_len, struct bvec_iter has bi_bvec_done, which represents the number of 20 bytes completed in the current bvec. 21 22 There are a bunch of new helper macros for hiding the gory details - in 23 particular, presenting the illusion of partially completed biovecs so that 24 normal code doesn't have to deal with bi_bvec_done. 25 26 * Driver code should no longer refer to biovecs directly; we now have 27 bio_iovec() and bio_iter_iovec() macros that return literal struct biovecs, 28 constructed from the raw biovecs but taking into account bi_bvec_done and 29 bi_size. 30 31 bio_for_each_segment() has been updated to take a bvec_iter argument 32 instead of an integer (that corresponded to bi_idx); for a lot of code the 33 conversion just required changing the types of the arguments to 34 bio_for_each_segment(). 35 36 * Advancing a bvec_iter is done with bio_advance_iter(); bio_advance() is a 37 wrapper around bio_advance_iter() that operates on bio->bi_iter, and also 38 advances the bio integrity's iter if present. 39 40 There is a lower level advance function - bvec_iter_advance() - which takes 41 a pointer to a biovec, not a bio; this is used by the bio integrity code. 42 43 What's all this get us? 44 ======================= 45 46 Having a real iterator, and making biovecs immutable, has a number of 47 advantages: 48 49 * Before, iterating over bios was very awkward when you weren't processing 50 exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c, 51 which copies the contents of one bio into another. Because the biovecs 52 wouldn't necessarily be the same size, the old code was tricky convoluted - 53 it had to walk two different bios at the same time, keeping both bi_idx and 54 and offset into the current biovec for each. 55 56 The new code is much more straightforward - have a look. This sort of 57 pattern comes up in a lot of places; a lot of drivers were essentially open 58 coding bvec iterators before, and having common implementation considerably 59 simplifies a lot of code. 60 61 * Before, any code that might need to use the biovec after the bio had been 62 completed (perhaps to copy the data somewhere else, or perhaps to resubmit 63 it somewhere else if there was an error) had to save the entire bvec array 64 - again, this was being done in a fair number of places. 65 66 * Biovecs can be shared between multiple bios - a bvec iter can represent an 67 arbitrary range of an existing biovec, both starting and ending midway 68 through biovecs. This is what enables efficient splitting of arbitrary 69 bios. Note that this means we _only_ use bi_size to determine when we've 70 reached the end of a bio, not bi_vcnt - and the bio_iovec() macro takes 71 bi_size into account when constructing biovecs. 72 73 * Splitting bios is now much simpler. The old bio_split() didn't even work on 74 bios with more than a single bvec! Now, we can efficiently split arbitrary 75 size bios - because the new bio can share the old bio's biovec. 76 77 Care must be taken to ensure the biovec isn't freed while the split bio is 78 still using it, in case the original bio completes first, though. Using 79 bio_chain() when splitting bios helps with this. 80 81 * Submitting partially completed bios is now perfectly fine - this comes up 82 occasionally in stacking block drivers and various code (e.g. md and 83 bcache) had some ugly workarounds for this. 84 85 It used to be the case that submitting a partially completed bio would work 86 fine to _most_ devices, but since accessing the raw bvec array was the 87 norm, not all drivers would respect bi_idx and those would break. Now, 88 since all drivers _must_ go through the bvec iterator - and have been 89 audited to make sure they are - submitting partially completed bios is 90 perfectly fine. 91 92 Other implications: 93 =================== 94 95 * Almost all usage of bi_idx is now incorrect and has been removed; instead, 96 where previously you would have used bi_idx you'd now use a bvec_iter, 97 probably passing it to one of the helper macros. 98 99 I.e. instead of using bio_iovec_idx() (or bio->bi_iovec[bio->bi_idx]), you 100 now use bio_iter_iovec(), which takes a bvec_iter and returns a 101 literal struct bio_vec - constructed on the fly from the raw biovec but 102 taking into account bi_bvec_done (and bi_size). 103 104 * bi_vcnt can't be trusted or relied upon by driver code - i.e. anything that 105 doesn't actually own the bio. The reason is twofold: firstly, it's not 106 actually needed for iterating over the bio anymore - we only use bi_size. 107 Secondly, when cloning a bio and reusing (a portion of) the original bio's 108 biovec, in order to calculate bi_vcnt for the new bio we'd have to iterate 109 over all the biovecs in the new bio - which is silly as it's not needed. 110 111 So, don't use bi_vcnt anymore. 112 113 * The current interface allows the block layer to split bios as needed, so we 114 could eliminate a lot of complexity particularly in stacked drivers. Code 115 that creates bios can then create whatever size bios are convenient, and 116 more importantly stacked drivers don't have to deal with both their own bio 117 size limitations and the limitations of the underlying devices. Thus 118 there's no need to define ->merge_bvec_fn() callbacks for individual block 119 drivers.