From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DD882C43334 for ; Mon, 27 Jun 2022 18:47:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239982AbiF0SrL (ORCPT ); Mon, 27 Jun 2022 14:47:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41904 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236776AbiF0SrK (ORCPT ); Mon, 27 Jun 2022 14:47:10 -0400 Received: from sin.source.kernel.org (sin.source.kernel.org [IPv6:2604:1380:40e1:4800::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DF831D53 for ; Mon, 27 Jun 2022 11:47:08 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by sin.source.kernel.org (Postfix) with ESMTPS id 17368CE1CB3 for ; Mon, 27 Jun 2022 18:47:07 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 83D24C3411D; Mon, 27 Jun 2022 18:47:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1656355625; bh=16rBFHUBxBBUynI8Jdzpix7xxMU+FNLty/Xeg/KzgjA=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=Sv4PtBSkO5jOTjjMwvIlyc3/8CR4aa/V+28r3eBxe+Kyl/cY6Py+WxlIOpIpCkO51 ZlcjzgEGP8X0+cmGBhPQtn8AzXyGO5AmByI2BAWHtmf3JBajkqpA3+pxYUQjKweE8w 0LtGx5rQ8vuewc/f7cpcjXEP29D19/xL+3oqtTr647SkZYsKFJvpDkH1Dz+PIZP378 wMGMvK8td3YkMEpupc+e1tyK2FvJZvmg6g0x3jGz0egEC6GyI3pJBjzwA0o85zoQMi 73uVpvTXbCRivFJIii/NuMrYlA6U01n+KqUgz3dS6OMSnXX735M2ek8hoG9uay6aZf XpRRN7jqESMbQ== Message-ID: <07ad7be25bab03c164bbd1f2d2264c9e6f79b70d.camel@kernel.org> Subject: Re: [PATCH 09/44] new iov_iter flavour - ITER_UBUF From: Jeff Layton To: Al Viro , linux-fsdevel@vger.kernel.org Cc: Linus Torvalds , Jens Axboe , Christoph Hellwig , Matthew Wilcox , David Howells , Dominique Martinet , Christian Brauner Date: Mon, 27 Jun 2022 14:47:03 -0400 In-Reply-To: <20220622041552.737754-9-viro@zeniv.linux.org.uk> References: <20220622041552.737754-1-viro@zeniv.linux.org.uk> <20220622041552.737754-9-viro@zeniv.linux.org.uk> Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.44.2 (3.44.2-1.fc36) MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, 2022-06-22 at 05:15 +0100, Al Viro wrote: > Equivalent of single-segment iovec. Initialized by iov_iter_ubuf(), > checked for by iter_is_ubuf(), otherwise behaves like ITER_IOVEC > ones. >=20 > We are going to expose the things like ->write_iter() et.al. to those > in subsequent commits. >=20 > New predicate (user_backed_iter()) that is true for ITER_IOVEC and > ITER_UBUF; places like direct-IO handling should use that for > checking that pages we modify after getting them from iov_iter_get_pages(= ) > would need to be dirtied. >=20 > DO NOT assume that replacing iter_is_iovec() with user_backed_iter() > will solve all problems - there's code that uses iter_is_iovec() to > decide how to poke around in iov_iter guts and for that the predicate > replacement obviously won't suffice. >=20 > Signed-off-by: Al Viro > --- > block/fops.c | 6 +-- > fs/ceph/file.c | 2 +- > fs/cifs/file.c | 2 +- > fs/direct-io.c | 2 +- > fs/fuse/dev.c | 4 +- > fs/fuse/file.c | 2 +- > fs/gfs2/file.c | 2 +- > fs/iomap/direct-io.c | 2 +- > fs/nfs/direct.c | 2 +- > include/linux/uio.h | 26 ++++++++++++ > lib/iov_iter.c | 94 ++++++++++++++++++++++++++++++++++---------- > mm/shmem.c | 2 +- > 12 files changed, 113 insertions(+), 33 deletions(-) >=20 > diff --git a/block/fops.c b/block/fops.c > index 6e86931ab847..3e68d69e0ee3 100644 > --- a/block/fops.c > +++ b/block/fops.c > @@ -69,7 +69,7 @@ static ssize_t __blkdev_direct_IO_simple(struct kiocb *= iocb, > =20 > if (iov_iter_rw(iter) =3D=3D READ) { > bio_init(&bio, bdev, vecs, nr_pages, REQ_OP_READ); > - if (iter_is_iovec(iter)) > + if (user_backed_iter(iter)) > should_dirty =3D true; > } else { > bio_init(&bio, bdev, vecs, nr_pages, dio_bio_write_op(iocb)); > @@ -199,7 +199,7 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb,= struct iov_iter *iter, > } > =20 > dio->size =3D 0; > - if (is_read && iter_is_iovec(iter)) > + if (is_read && user_backed_iter(iter)) > dio->flags |=3D DIO_SHOULD_DIRTY; > =20 > blk_start_plug(&plug); > @@ -331,7 +331,7 @@ static ssize_t __blkdev_direct_IO_async(struct kiocb = *iocb, > dio->size =3D bio->bi_iter.bi_size; > =20 > if (is_read) { > - if (iter_is_iovec(iter)) { > + if (user_backed_iter(iter)) { > dio->flags |=3D DIO_SHOULD_DIRTY; > bio_set_pages_dirty(bio); > } > diff --git a/fs/ceph/file.c b/fs/ceph/file.c > index 8c8226c0feac..e132adeeaf16 100644 > --- a/fs/ceph/file.c > +++ b/fs/ceph/file.c > @@ -1262,7 +1262,7 @@ ceph_direct_read_write(struct kiocb *iocb, struct i= ov_iter *iter, > size_t count =3D iov_iter_count(iter); > loff_t pos =3D iocb->ki_pos; > bool write =3D iov_iter_rw(iter) =3D=3D WRITE; > - bool should_dirty =3D !write && iter_is_iovec(iter); > + bool should_dirty =3D !write && user_backed_iter(iter); > =20 > if (write && ceph_snap(file_inode(file)) !=3D CEPH_NOSNAP) > return -EROFS; > diff --git a/fs/cifs/file.c b/fs/cifs/file.c > index 1618e0537d58..4b4129d9a90c 100644 > --- a/fs/cifs/file.c > +++ b/fs/cifs/file.c > @@ -4004,7 +4004,7 @@ static ssize_t __cifs_readv( > if (!is_sync_kiocb(iocb)) > ctx->iocb =3D iocb; > =20 > - if (iter_is_iovec(to)) > + if (user_backed_iter(to)) > ctx->should_dirty =3D true; > =20 > if (direct) { > diff --git a/fs/direct-io.c b/fs/direct-io.c > index 39647eb56904..72237f49ad94 100644 > --- a/fs/direct-io.c > +++ b/fs/direct-io.c > @@ -1245,7 +1245,7 @@ ssize_t __blockdev_direct_IO(struct kiocb *iocb, st= ruct inode *inode, > spin_lock_init(&dio->bio_lock); > dio->refcount =3D 1; > =20 > - dio->should_dirty =3D iter_is_iovec(iter) && iov_iter_rw(iter) =3D=3D R= EAD; > + dio->should_dirty =3D user_backed_iter(iter) && iov_iter_rw(iter) =3D= =3D READ; > sdio.iter =3D iter; > sdio.final_block_in_request =3D end >> blkbits; > =20 > diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c > index 0e537e580dc1..8d657c2cd6f7 100644 > --- a/fs/fuse/dev.c > +++ b/fs/fuse/dev.c > @@ -1356,7 +1356,7 @@ static ssize_t fuse_dev_read(struct kiocb *iocb, st= ruct iov_iter *to) > if (!fud) > return -EPERM; > =20 > - if (!iter_is_iovec(to)) > + if (!user_backed_iter(to)) > return -EINVAL; > =20 > fuse_copy_init(&cs, 1, to); > @@ -1949,7 +1949,7 @@ static ssize_t fuse_dev_write(struct kiocb *iocb, s= truct iov_iter *from) > if (!fud) > return -EPERM; > =20 > - if (!iter_is_iovec(from)) > + if (!user_backed_iter(from)) > return -EINVAL; > =20 > fuse_copy_init(&cs, 0, from); > diff --git a/fs/fuse/file.c b/fs/fuse/file.c > index 00fa861aeead..c982e3afe3b4 100644 > --- a/fs/fuse/file.c > +++ b/fs/fuse/file.c > @@ -1465,7 +1465,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, str= uct iov_iter *iter, > inode_unlock(inode); > } > =20 > - io->should_dirty =3D !write && iter_is_iovec(iter); > + io->should_dirty =3D !write && user_backed_iter(iter); > while (count) { > ssize_t nres; > fl_owner_t owner =3D current->files; > diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c > index 2cceb193dcd8..48e6cc74fdc1 100644 > --- a/fs/gfs2/file.c > +++ b/fs/gfs2/file.c > @@ -780,7 +780,7 @@ static inline bool should_fault_in_pages(struct iov_i= ter *i, > =20 > if (!count) > return false; > - if (!iter_is_iovec(i)) > + if (!user_backed_iter(i)) > return false; > =20 > size =3D PAGE_SIZE; > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c > index 31c7f1035b20..d5c7d019653b 100644 > --- a/fs/iomap/direct-io.c > +++ b/fs/iomap/direct-io.c > @@ -533,7 +533,7 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *i= ter, > iomi.flags |=3D IOMAP_NOWAIT; > } > =20 > - if (iter_is_iovec(iter)) > + if (user_backed_iter(iter)) > dio->flags |=3D IOMAP_DIO_DIRTY; > } else { > iomi.flags |=3D IOMAP_WRITE; > diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c > index 4eb2a8380a28..022e1ce63e62 100644 > --- a/fs/nfs/direct.c > +++ b/fs/nfs/direct.c > @@ -478,7 +478,7 @@ ssize_t nfs_file_direct_read(struct kiocb *iocb, stru= ct iov_iter *iter, > if (!is_sync_kiocb(iocb)) > dreq->iocb =3D iocb; > =20 > - if (iter_is_iovec(iter)) > + if (user_backed_iter(iter)) > dreq->flags =3D NFS_ODIRECT_SHOULD_DIRTY; > =20 > if (!swap) > diff --git a/include/linux/uio.h b/include/linux/uio.h > index 76d305f3d4c2..6ab4260c3d6c 100644 > --- a/include/linux/uio.h > +++ b/include/linux/uio.h > @@ -26,6 +26,7 @@ enum iter_type { > ITER_PIPE, > ITER_XARRAY, > ITER_DISCARD, > + ITER_UBUF, > }; > =20 > struct iov_iter_state { > @@ -38,6 +39,7 @@ struct iov_iter { > u8 iter_type; > bool nofault; > bool data_source; > + bool user_backed; > size_t iov_offset; > size_t count; > union { > @@ -46,6 +48,7 @@ struct iov_iter { > const struct bio_vec *bvec; > struct xarray *xarray; > struct pipe_inode_info *pipe; > + void __user *ubuf; > }; > union { > unsigned long nr_segs; > @@ -70,6 +73,11 @@ static inline void iov_iter_save_state(struct iov_iter= *iter, > state->nr_segs =3D iter->nr_segs; > } > =20 > +static inline bool iter_is_ubuf(const struct iov_iter *i) > +{ > + return iov_iter_type(i) =3D=3D ITER_UBUF; > +} > + > static inline bool iter_is_iovec(const struct iov_iter *i) > { > return iov_iter_type(i) =3D=3D ITER_IOVEC; > @@ -105,6 +113,11 @@ static inline unsigned char iov_iter_rw(const struct= iov_iter *i) > return i->data_source ? WRITE : READ; > } > =20 > +static inline bool user_backed_iter(const struct iov_iter *i) > +{ > + return i->user_backed; > +} > + nit: I wonder whether this new boolean is worth it over just checking is_iter_iovec() || is_iter_ubuf. Not a big deal though. > /* > * Total number of bytes covered by an iovec. > * > @@ -320,4 +333,17 @@ ssize_t __import_iovec(int type, const struct iovec = __user *uvec, > int import_single_range(int type, void __user *buf, size_t len, > struct iovec *iov, struct iov_iter *i); > =20 > +static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direct= ion, > + void __user *buf, size_t count) > +{ > + WARN_ON(direction & ~(READ | WRITE)); > + *i =3D (struct iov_iter) { > + .iter_type =3D ITER_UBUF, > + .user_backed =3D true, > + .data_source =3D direction, > + .ubuf =3D buf, > + .count =3D count > + }; > +} > + > #endif > diff --git a/lib/iov_iter.c b/lib/iov_iter.c > index 4c658a25e29c..8275b28e886b 100644 > --- a/lib/iov_iter.c > +++ b/lib/iov_iter.c > @@ -16,6 +16,16 @@ > =20 > #define PIPE_PARANOIA /* for now */ > =20 > +/* covers ubuf and kbuf alike */ > +#define iterate_buf(i, n, base, len, off, __p, STEP) { \ > + size_t __maybe_unused off =3D 0; \ > + len =3D n; \ > + base =3D __p + i->iov_offset; \ > + len -=3D (STEP); \ > + i->iov_offset +=3D len; \ > + n =3D len; \ > +} > + > /* covers iovec and kvec alike */ > #define iterate_iovec(i, n, base, len, off, __p, STEP) { \ > size_t off =3D 0; \ > @@ -110,7 +120,12 @@ __out: \ > if (unlikely(i->count < n)) \ > n =3D i->count; \ > if (likely(n)) { \ > - if (likely(iter_is_iovec(i))) { \ > + if (likely(iter_is_ubuf(i))) { \ > + void __user *base; \ > + size_t len; \ > + iterate_buf(i, n, base, len, off, \ > + i->ubuf, (I)) \ > + } else if (likely(iter_is_iovec(i))) { \ > const struct iovec *iov =3D i->iov; \ > void __user *base; \ > size_t len; \ > @@ -275,7 +290,11 @@ static size_t copy_page_to_iter_pipe(struct page *pa= ge, size_t offset, size_t by > */ > size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t size) > { > - if (iter_is_iovec(i)) { > + if (iter_is_ubuf(i)) { > + size_t n =3D min(size, iov_iter_count(i)); > + n -=3D fault_in_readable(i->ubuf + i->iov_offset, n); > + return size - n; > + } else if (iter_is_iovec(i)) { > size_t count =3D min(size, iov_iter_count(i)); > const struct iovec *p; > size_t skip; > @@ -314,7 +333,11 @@ EXPORT_SYMBOL(fault_in_iov_iter_readable); > */ > size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t size= ) > { > - if (iter_is_iovec(i)) { > + if (iter_is_ubuf(i)) { > + size_t n =3D min(size, iov_iter_count(i)); > + n -=3D fault_in_safe_writeable(i->ubuf + i->iov_offset, n); > + return size - n; > + } else if (iter_is_iovec(i)) { > size_t count =3D min(size, iov_iter_count(i)); > const struct iovec *p; > size_t skip; > @@ -345,6 +368,7 @@ void iov_iter_init(struct iov_iter *i, unsigned int d= irection, > *i =3D (struct iov_iter) { > .iter_type =3D ITER_IOVEC, > .nofault =3D false, > + .user_backed =3D true, > .data_source =3D direction, > .iov =3D iov, > .nr_segs =3D nr_segs, > @@ -494,7 +518,7 @@ size_t _copy_to_iter(const void *addr, size_t bytes, = struct iov_iter *i) > { > if (unlikely(iov_iter_is_pipe(i))) > return copy_pipe_to_iter(addr, bytes, i); > - if (iter_is_iovec(i)) > + if (user_backed_iter(i)) > might_fault(); > iterate_and_advance(i, bytes, base, len, off, > copyout(base, addr + off, len), > @@ -576,7 +600,7 @@ size_t _copy_mc_to_iter(const void *addr, size_t byte= s, struct iov_iter *i) > { > if (unlikely(iov_iter_is_pipe(i))) > return copy_mc_pipe_to_iter(addr, bytes, i); > - if (iter_is_iovec(i)) > + if (user_backed_iter(i)) > might_fault(); > __iterate_and_advance(i, bytes, base, len, off, > copyout_mc(base, addr + off, len), > @@ -594,7 +618,7 @@ size_t _copy_from_iter(void *addr, size_t bytes, stru= ct iov_iter *i) > WARN_ON(1); > return 0; > } > - if (iter_is_iovec(i)) > + if (user_backed_iter(i)) > might_fault(); > iterate_and_advance(i, bytes, base, len, off, > copyin(addr + off, base, len), > @@ -882,16 +906,16 @@ void iov_iter_advance(struct iov_iter *i, size_t si= ze) > { > if (unlikely(i->count < size)) > size =3D i->count; > - if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i))) { > + if (likely(iter_is_ubuf(i)) || unlikely(iov_iter_is_xarray(i))) { > + i->iov_offset +=3D size; > + i->count -=3D size; > + } else if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i))) { > /* iovec and kvec have identical layouts */ > iov_iter_iovec_advance(i, size); > } else if (iov_iter_is_bvec(i)) { > iov_iter_bvec_advance(i, size); > } else if (iov_iter_is_pipe(i)) { > pipe_advance(i, size); > - } else if (unlikely(iov_iter_is_xarray(i))) { > - i->iov_offset +=3D size; > - i->count -=3D size; > } else if (iov_iter_is_discard(i)) { > i->count -=3D size; > } > @@ -938,7 +962,7 @@ void iov_iter_revert(struct iov_iter *i, size_t unrol= l) > return; > } > unroll -=3D i->iov_offset; > - if (iov_iter_is_xarray(i)) { > + if (iov_iter_is_xarray(i) || iter_is_ubuf(i)) { > BUG(); /* We should never go beyond the start of the specified > * range since we might then be straying into pages that > * aren't pinned. > @@ -1129,6 +1153,13 @@ static unsigned long iov_iter_alignment_bvec(const= struct iov_iter *i) > =20 > unsigned long iov_iter_alignment(const struct iov_iter *i) > { > + if (likely(iter_is_ubuf(i))) { > + size_t size =3D i->count; > + if (size) > + return ((unsigned long)i->ubuf + i->iov_offset) | size; > + return 0; > + } > + > /* iovec and kvec have identical layouts */ > if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i))) > return iov_iter_alignment_iovec(i); > @@ -1159,6 +1190,9 @@ unsigned long iov_iter_gap_alignment(const struct i= ov_iter *i) > size_t size =3D i->count; > unsigned k; > =20 > + if (iter_is_ubuf(i)) > + return 0; > + > if (WARN_ON(!iter_is_iovec(i))) > return ~0U; > =20 > @@ -1287,7 +1321,19 @@ static ssize_t iter_xarray_get_pages(struct iov_it= er *i, > return actual; > } > =20 > -/* must be done on non-empty ITER_IOVEC one */ > +static unsigned long found_ubuf_segment(unsigned long addr, > + size_t len, > + size_t *size, size_t *start, > + unsigned maxpages) > +{ > + len +=3D (*start =3D addr % PAGE_SIZE); > + if (len > maxpages * PAGE_SIZE) > + len =3D maxpages * PAGE_SIZE; > + *size =3D len; > + return addr & PAGE_MASK; > +} > + > +/* must be done on non-empty ITER_UBUF or ITER_IOVEC one */ > static unsigned long first_iovec_segment(const struct iov_iter *i, > size_t *size, size_t *start, > size_t maxsize, unsigned maxpages) > @@ -1295,6 +1341,11 @@ static unsigned long first_iovec_segment(const str= uct iov_iter *i, > size_t skip; > long k; > =20 > + if (iter_is_ubuf(i)) { > + unsigned long addr =3D (unsigned long)i->ubuf + i->iov_offset; > + return found_ubuf_segment(addr, maxsize, size, start, maxpages); > + } > + > for (k =3D 0, skip =3D i->iov_offset; k < i->nr_segs; k++, skip =3D 0) = { > unsigned long addr =3D (unsigned long)i->iov[k].iov_base + skip; > size_t len =3D i->iov[k].iov_len - skip; > @@ -1303,11 +1354,7 @@ static unsigned long first_iovec_segment(const str= uct iov_iter *i, > continue; > if (len > maxsize) > len =3D maxsize; > - len +=3D (*start =3D addr % PAGE_SIZE); > - if (len > maxpages * PAGE_SIZE) > - len =3D maxpages * PAGE_SIZE; > - *size =3D len; > - return addr & PAGE_MASK; > + return found_ubuf_segment(addr, len, size, start, maxpages); > } > BUG(); // if it had been empty, we wouldn't get called > } > @@ -1344,7 +1391,7 @@ ssize_t iov_iter_get_pages(struct iov_iter *i, > if (!maxsize) > return 0; > =20 > - if (likely(iter_is_iovec(i))) { > + if (likely(user_backed_iter(i))) { > unsigned int gup_flags =3D 0; > unsigned long addr; > =20 > @@ -1470,7 +1517,7 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i= , > if (!maxsize) > return 0; > =20 > - if (likely(iter_is_iovec(i))) { > + if (likely(user_backed_iter(i))) { > unsigned int gup_flags =3D 0; > unsigned long addr; > =20 > @@ -1624,6 +1671,11 @@ int iov_iter_npages(const struct iov_iter *i, int = maxpages) > { > if (unlikely(!i->count)) > return 0; > + if (likely(iter_is_ubuf(i))) { > + unsigned offs =3D offset_in_page(i->ubuf + i->iov_offset); > + int npages =3D DIV_ROUND_UP(offs + i->count, PAGE_SIZE); > + return min(npages, maxpages); > + } > /* iovec and kvec have identical layouts */ > if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i))) > return iov_npages(i, maxpages); > @@ -1862,10 +1914,12 @@ EXPORT_SYMBOL(import_single_range); > void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state) > { > if (WARN_ON_ONCE(!iov_iter_is_bvec(i) && !iter_is_iovec(i)) && > - !iov_iter_is_kvec(i)) > + !iov_iter_is_kvec(i) && !iter_is_ubuf(i)) > return; > i->iov_offset =3D state->iov_offset; > i->count =3D state->count; > + if (iter_is_ubuf(i)) > + return; > /* > * For the *vec iters, nr_segs + iov is constant - if we increment > * the vec, then we also decrement the nr_segs count. Hence we don't > diff --git a/mm/shmem.c b/mm/shmem.c > index a6f565308133..6b83f3971795 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -2603,7 +2603,7 @@ static ssize_t shmem_file_read_iter(struct kiocb *i= ocb, struct iov_iter *to) > ret =3D copy_page_to_iter(page, offset, nr, to); > put_page(page); > =20 > - } else if (iter_is_iovec(to)) { > + } else if (!user_backed_iter(to)) { > /* > * Copy to user tends to be so well optimized, but > * clear_user() not so much, that it is noticeably The code looks reasonable but is there any real benefit here? It seems like the only user of it so far is new_sync_{read,write}, and both seem to just use it to avoid allocating a single iovec on the stack. =20 --=20 Jeff Layton