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Commit 875f1d07 authored by Jens Axboe's avatar Jens Axboe
Browse files

iov_iter: add ITER_BVEC_FLAG_NO_REF flag 



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: default avatarJens Axboe <axboe@kernel.dk>
parent bf33a769

fs/io_uring.c

+3 −0
Original line number Diff line number Diff line
@@ -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;

}

include/linux/uio.h

+19 −5
Original line number Diff line number Diff line
@@ -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.

 *