RE: [PATCH 2/2] block: no-copy bvec for direct IO
From: David Laight <hidden>
Date: 2020-12-09 21:16:20
Also in:
io-uring, linux-fsdevel, lkml
From: Pavel Begunkov
Sent: 09 December 2020 02:20 The block layer spends quite a while in blkdev_direct_IO() to copy and initialise bio's bvec. However, if we've already got a bvec in the input iterator it might be reused in some cases, i.e. when new ITER_BVEC_FLAG_FIXED flag is set. Simple tests show considerable performance boost, and it also reduces memory footprint.
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quoted hunk ↗ jump to hunk
@@ -398,7 +422,11 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_vecs) bio->bi_end_io = blkdev_bio_end_io; bio->bi_ioprio = iocb->ki_ioprio; - ret = bio_iov_iter_get_pages(bio, iter); + if (iov_iter_is_bvec(iter) && iov_iter_bvec_fixed(iter)) + ret = bio_iov_fixed_bvec_get_pages(bio, iter); + else + ret = bio_iov_iter_get_pages(bio, iter); +
Is it necessary to check iov_iter_is_bvec() as well as iov_iter_bvec_fixed() ? If so it is probably worth using & not && so the compiler stands a chance of generating a & (B | C) == B instead of 2 conditionals. (I think I saw the bits in the same field being tested. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)