Messages in this thread | | | Date | Thu, 9 Nov 2023 17:41:42 +0000 | Subject | Re: [PATCH 02/21] block: Limit atomic writes according to bio and queue limits | From | John Garry <> |
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On 09/11/2023 15:13, Christoph Hellwig wrote: > On Fri, Sep 29, 2023 at 10:27:07AM +0000, John Garry wrote: >> We rely the block layer always being able to send a bio of size >> atomic_write_unit_max without being required to split it due to request >> queue or other bio limits. >> >> A bio may contain min(BIO_MAX_VECS, limits->max_segments) vectors, >> and each vector is at worst case the device logical block size from >> direct IO alignment requirement. > A bio can have more than BIO_MAX_VECS if you use bio_init.
Right, FWIW we are only concerned with codepaths which use BIO_MAX_VECS, but I suppose that is not good enough as a guarantee.
> >> +static unsigned int blk_queue_max_guaranteed_bio_size_sectors( >> + struct request_queue *q) >> +{ >> + struct queue_limits *limits = &q->limits; >> + unsigned int max_segments = min_t(unsigned int, BIO_MAX_VECS, >> + limits->max_segments); >> + /* Limit according to dev sector size as we only support direct-io */ > Who is "we", and how tells the caller to only ever use direct I/O?
I think that this can be dropped as a comment. My earlier series used PAGE_SIZE and not sector size here, which I think was proper.
> And how would a type of userspace I/O even matter for low-level > block code.
It shouldn't do, but we still need to limit according to request queue limits.
> What if I wanted to use this for file system metadata? >
As mentioned, I think that the direct-IO comment can be dropped.
Thanks, John
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