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iov_iter: add ITER_BVEC_FLAG_NO_REF flag

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For ITER_BVEC, if we're holding on to kernel pages, the caller
doesn't need to grab a reference to the bvec pages, and drop that
same reference on IO completion. This is essentially safe for any
ITER_BVEC, but some use cases end up reusing pages and uncondtionally
dropping a page reference on completion. And example of that is
sendfile(2), that ends up being a splice_in + splice_out on the
pipe pages.

Add a flag that tells us it's fine to not grab a page reference
to the bvec pages, since that caller knows not to drop a reference
when it's done with the pages.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2019-02-27 13:05:25 -07:00
parent bf33a7699e
commit 875f1d0769
2 changed files with 22 additions and 5 deletions
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3
fs/io_uring.c
View File

@ -855,6 +855,9 @@ static int io_import_fixed(struct io_ring_ctx *ctx, int rw,
iov_iter_bvec(iter, rw, imu->bvec, imu->nr_bvecs, offset + len);
if (offset)
iov_iter_advance(iter, offset);
/* don't drop a reference to these pages */
iter->type |= ITER_BVEC_FLAG_NO_REF;
return 0;
}

24
include/linux/uio.h
View File

@ -23,14 +23,23 @@ struct kvec {
};
enum iter_type {
ITER_IOVEC = 0,
ITER_KVEC = 2,
ITER_BVEC = 4,
ITER_PIPE = 8,
ITER_DISCARD = 16,
/* set if ITER_BVEC doesn't hold a bv_page ref */
ITER_BVEC_FLAG_NO_REF = 2,
/* iter types */
ITER_IOVEC = 4,
ITER_KVEC = 8,
ITER_BVEC = 16,
ITER_PIPE = 32,
ITER_DISCARD = 64,
};
struct iov_iter {
/*
* Bit 0 is the read/write bit, set if we're writing.
* Bit 1 is the BVEC_FLAG_NO_REF bit, set if type is a bvec and
* the caller isn't expecting to drop a page reference when done.
*/
unsigned int type;
size_t iov_offset;
size_t count;
@ -84,6 +93,11 @@ static inline unsigned char iov_iter_rw(const struct iov_iter *i)
return i->type & (READ | WRITE);
}
static inline bool iov_iter_bvec_no_ref(const struct iov_iter *i)
{
return (i->type & ITER_BVEC_FLAG_NO_REF) != 0;
}
/*
* Total number of bytes covered by an iovec.
*