Re: [PATCH 02/21] block: Limit atomic writes according to bio and queue limits

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


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.


[Index of Archives]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Yosemite News]     [Linux Kernel]     [Linux SCSI]

  Powered by Linux