Messages in this thread | | | From | Chris Mason <> | Subject | Re: [PATCH] block: Revert bio_clone() default behaviour | Date | Wed, 6 Nov 2013 11:11:30 -0500 |
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Quoting Kent Overstreet (2013-11-05 22:48:41) > This patch reverts the default behaviour introduced by > 9fc6286f347d00528adcdcf12396d220f47492ed - bio_clone_biovec() no clonger > shares the source bio's biovec, cloning the biovec is once again the > default. > > Instead, we add a new bio_clone_biovec_fast(), which creates a clone > that shares the source's biovec. This patch changes bcache and md to use ^^^^^ dm?
> __bio_clone_biovec_fast() since they're expecting the new behaviour due > to other refactoring; most of the other uses of bio_clone() should be > same to convert to the _fast() variant but that will be done more > incrementally in other patches (bio_split() in particular).
Hi Kent,
I noticed yesterday the _fast variants of bio clone introduce sharing between the src and the clone, but without any reference counts:
bio->bi_io_vec = bio_src->bi_io_vec;
Have you audited all of the _fast users to make sure they are not freeing the src before the clone? Sorry if this came up already in past reviews.
> > Note that __bio_clone() isn't being readded - the reason being that with > immutable biovecs allocating the right number of biovecs for the new > clone is no longer trivial so we don't want drivers trying to do that > themselves. > > This patch also reverts febca1baea1cfe2d7a0271385d89b03d5fb34f94 - > __bio_clone_fast() should not be setting bi_vcnt for bios that do not > own the biovec (see Documentation/block/biovecs.txt for rationale) - in > short,
I think I see what you mean with tying bi_vcnt to ownership of the bio, but we're not consistent. Looking at bio_for_eaach_segment_all:
* * drivers should _never_ use the all version - the bio may have been split * before it got to the driver and the driver won't own all of it */ #define bio_for_each_segment_all(bvl, bio, i) \ for (i = 0, bvl = (bio)->bi_io_vec; i < (bio)->bi_vcnt; i++, bvl++)
bio_for_each_segment_all still trusts bi_vcnt, so any bio_for_each_segment_all operation on a clone will basically be a noop.
Just looking at MD raid1 make_request()
mbio = bio_clone_mddev(bio, GFP_NOIO, mddev); ... alloc_behind_pages(mbio, r1_bio); -> bio_for_each_segment_all ... if (r1_bio->behind_bvecs) { bio_for_each_segment_all(bvec, mbio, j) ...
I didn't test MD without the vcnt fix, but I think any operations in MD that duplicate data for raid1 turn into noops. I think we'll end up writing garbage (or nothing) to the second mirror.
If you look at dm's crypt_free_buffer_pages(), it had similar problems.
> not setting it might cause bugs in the short term but long term > it's likely to hide nastier more subtle bugs, we don't want code looking > at bi_vcnt at all for bios it does not own.
I think the concept of bio ownership is still much too weak, at least for established users like MD and DM. I don't know how to verify the sharing of bi_io_vec without some kind of reference counting on the iovec.
-chris
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