[PATCH v2 3/3] bio: convert get_user_pages_fast() --> pin_user_pages_fast()

John Hubbard
2020-08-29T08:09:18Z

Change generic block/bio Direct IO routines, to acquire FOLL_PIN user
pages via the recently added routines:

iov_iter_pin_user_pages()
iov_iter_pin_user_pages_alloc()
pin_user_page()

This effectively converts several file systems (ext4, for example) that
use the common Direct IO routines.

Change the corresponding page release calls from put_page() to
unpin_user_page().

Change bio_release_pages() to handle FOLL_PIN pages. In fact, that
is now the *only* type of pages it handles now.

Design notes
============

Quite a few approaches have been considered over the years. This one is
inspired by Christoph Hellwig's July, 2019 observation that there are
only 5 ITER_ types, and we can simplify handling of them for Direct IO
[1]. Accordingly, this patch implements the following pseudocode:

Direct IO behavior:

ITER_IOVEC:
pin_user_pages_fast();
break;

ITER_PIPE:
for each page:
pin_user_page();
break;

ITER_KVEC: // already elevated page refcount, leave alone
ITER_BVEC: // already elevated page refcount, leave alone
ITER_DISCARD: // discard
return -EFAULT or -ENVALID;

...which works for callers that already have sorted out which case they
are in. Such as, Direct IO in the block/bio layers.

Now, this does leave ITER_KVEC and ITER_BVEC unconverted, but on the
other hand, it's not clear that these are actually affected in the real
world, by the get_user_pages()+filesystem interaction problems of [2].
If it turns out to matter, then those can be handled too, but it's just
more refactoring and surgery to do so.

Page acquisition: The iov_iter_get_pages*() routines
above are at just the right level in the call stack: the callers already
know which system to use, and so it's a small change to just drop in the
replacement routines. And it's a fan-in/fan-out point: block/bio call
sites for Direct IO funnel their page acquisitions through the
iov_iter_get_pages*() routines, and there are many other callers of
those. And we can't convert all of the callers at once--too many
subsystems are involved, and it would be a too large and too risky
patch.

Page release: there are already separate release routines: put_page()
vs. unpin_user_page(), so it's already done there.

[1] https://lore.kernel.org/kvm/20190724061750.GA19397@infradead.org/

[2] "Explicit pinning of user-space pages":
https://lwn.net/Articles/807108/

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
block/bio.c | 24 ++++++++++++------------
block/blk-map.c | 6 +++---
fs/direct-io.c | 28 ++++++++++++++--------------
fs/iomap/direct-io.c | 2 +-
4 files changed, 30 insertions(+), 30 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index a9931f23d933..f54e9414e6d9 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -955,7 +955,7 @@ void bio_release_pages(struct bio *bio, bool mark_dirty)
bio_for_each_segment_all(bvec, bio, iter_all) {
if (mark_dirty && !PageCompound(bvec->bv_page))
set_page_dirty_lock(bvec->bv_page);
- put_page(bvec->bv_page);
+ unpin_user_page(bvec->bv_page);
}
}
EXPORT_SYMBOL_GPL(bio_release_pages);
@@ -986,9 +986,9 @@ static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
* @iter: iov iterator describing the region to be mapped
*
* Pins pages from *iter and appends them to @bio's bvec array. The
- * pages will have to be released using put_page() when done.
- * For multi-segment *iter, this function only adds pages from the
- * next non-empty segment of the iov iterator.
+ * pages will have to be released using put_page() or unpin_user_page() when
+ * done. For multi-segment *iter, this function only adds pages from the next
+ * non-empty segment of the iov iterator.
*/
static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
{
@@ -1009,7 +1009,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);

- size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
+ size = iov_iter_pin_user_pages(iter, pages, LONG_MAX, nr_pages, &offset);
if (unlikely(size <= 0))
return size ? size : -EFAULT;

@@ -1020,7 +1020,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)

if (__bio_try_merge_page(bio, page, len, offset, &same_page)) {
if (same_page)
- put_page(page);
+ unpin_user_page(page);
} else {
if (WARN_ON_ONCE(bio_full(bio, len)))
return -EINVAL;
@@ -1056,7 +1056,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);

- size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
+ size = iov_iter_pin_user_pages(iter, pages, LONG_MAX, nr_pages, &offset);
if (unlikely(size <= 0))
return size ? size : -EFAULT;

@@ -1069,7 +1069,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
max_append_sectors, &same_page) != len)
return -EINVAL;
if (same_page)
- put_page(page);
+ unpin_user_page(page);
offset = 0;
}

@@ -1113,8 +1113,8 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
} else {
if (is_bvec)
ret = __bio_iov_bvec_add_pages(bio, iter);
- else
- ret = __bio_iov_iter_get_pages(bio, iter);
+ else
+ ret = __bio_iov_iter_get_pages(bio, iter);
}
} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));

@@ -1326,8 +1326,8 @@ void bio_set_pages_dirty(struct bio *bio)
* the BIO and re-dirty the pages in process context.
*
* It is expected that bio_check_pages_dirty() will wholly own the BIO from
- * here on. It will run one put_page() against each page and will run one
- * bio_put() against the BIO.
+ * here on. It will run one unpin_user_page() against each page
+ * and will run one bio_put() against the BIO.
*/

static void bio_dirty_fn(struct work_struct *work);
diff --git a/block/blk-map.c b/block/blk-map.c
index 6e804892d5ec..7a095b4947ea 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -275,7 +275,7 @@ static struct bio *bio_map_user_iov(struct request_queue *q,
size_t offs, added = 0;
int npages;

- bytes = iov_iter_get_pages_alloc(iter, &pages, LONG_MAX, &offs);
+ bytes = iov_iter_pin_user_pages_alloc(iter, &pages, LONG_MAX, &offs);
if (unlikely(bytes <= 0)) {
ret = bytes ? bytes : -EFAULT;
goto out_unmap;
@@ -298,7 +298,7 @@ static struct bio *bio_map_user_iov(struct request_queue *q,
if (!bio_add_hw_page(q, bio, page, n, offs,
max_sectors, &same_page)) {
if (same_page)
- put_page(page);
+ unpin_user_page(page);
break;
}

@@ -312,7 +312,7 @@ static struct bio *bio_map_user_iov(struct request_queue *q,
* release the pages we didn't map into the bio, if any
*/
while (j < npages)
- put_page(pages[j++]);
+ unpin_user_page(pages[j++]);
kvfree(pages);
/* couldn't stuff something into bio? */
if (bytes)
diff --git a/fs/direct-io.c b/fs/direct-io.c
index 183299892465..b01c8d003bd3 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -170,7 +170,7 @@ static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
{
ssize_t ret;

- ret = iov_iter_get_pages(sdio->iter, dio->pages, LONG_MAX, DIO_PAGES,
+ ret = iov_iter_pin_user_pages(sdio->iter, dio->pages, LONG_MAX, DIO_PAGES,
&sdio->from);

if (ret < 0 && sdio->blocks_available && (dio->op == REQ_OP_WRITE)) {
@@ -182,7 +182,7 @@ static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
*/
if (dio->page_errors == 0)
dio->page_errors = ret;
- get_page(page);
+ pin_user_page(page);
dio->pages[0] = page;
sdio->head = 0;
sdio->tail = 1;
@@ -472,7 +472,7 @@ static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
static inline void dio_cleanup(struct dio *dio, struct dio_submit *sdio)
{
while (sdio->head < sdio->tail)
- put_page(dio->pages[sdio->head++]);
+ unpin_user_page(dio->pages[sdio->head++]);
}

/*
@@ -739,7 +739,7 @@ static inline int dio_bio_add_page(struct dio_submit *sdio)
*/
if ((sdio->cur_page_len + sdio->cur_page_offset) == PAGE_SIZE)
sdio->pages_in_io--;
- get_page(sdio->cur_page);
+ pin_user_page(sdio->cur_page);
sdio->final_block_in_bio = sdio->cur_page_block +
(sdio->cur_page_len >> sdio->blkbits);
ret = 0;
@@ -853,13 +853,13 @@ submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
*/
if (sdio->cur_page) {
ret = dio_send_cur_page(dio, sdio, map_bh);
- put_page(sdio->cur_page);
+ unpin_user_page(sdio->cur_page);
sdio->cur_page = NULL;
if (ret)
return ret;
}

- get_page(page); /* It is in dio */
+ pin_user_page(page); /* It is in dio */
sdio->cur_page = page;
sdio->cur_page_offset = offset;
sdio->cur_page_len = len;
@@ -874,7 +874,7 @@ submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
ret = dio_send_cur_page(dio, sdio, map_bh);
if (sdio->bio)
dio_bio_submit(dio, sdio);
- put_page(sdio->cur_page);
+ unpin_user_page(sdio->cur_page);
sdio->cur_page = NULL;
}
return ret;
@@ -974,7 +974,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,

ret = get_more_blocks(dio, sdio, map_bh);
if (ret) {
- put_page(page);
+ unpin_user_page(page);
goto out;
}
if (!buffer_mapped(map_bh))
@@ -1019,7 +1019,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,

/* AKPM: eargh, -ENOTBLK is a hack */
if (dio->op == REQ_OP_WRITE) {
- put_page(page);
+ unpin_user_page(page);
return -ENOTBLK;
}

@@ -1032,7 +1032,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
if (sdio->block_in_file >=
i_size_aligned >> blkbits) {
/* We hit eof */
- put_page(page);
+ unpin_user_page(page);
goto out;
}
zero_user(page, from, 1 << blkbits);
@@ -1072,7 +1072,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
sdio->next_block_for_io,
map_bh);
if (ret) {
- put_page(page);
+ unpin_user_page(page);
goto out;
}
sdio->next_block_for_io += this_chunk_blocks;
@@ -1087,8 +1087,8 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
break;
}

- /* Drop the ref which was taken in get_user_pages() */
- put_page(page);
+ /* Drop the ref which was taken in pin_user_pages() */
+ unpin_user_page(page);
}
out:
return ret;
@@ -1327,7 +1327,7 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
ret2 = dio_send_cur_page(dio, &sdio, &map_bh);
if (retval == 0)
retval = ret2;
- put_page(sdio.cur_page);
+ unpin_user_page(sdio.cur_page);
sdio.cur_page = NULL;
}
if (sdio.bio)
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c1aafb2ab990..390f611528ea 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -194,7 +194,7 @@ iomap_dio_zero(struct iomap_dio *dio, struct iomap *iomap, loff_t pos,
bio->bi_private = dio;
bio->bi_end_io = iomap_dio_bio_end_io;

- get_page(page);
+ pin_user_page(page);
__bio_add_page(bio, page, len, 0);
bio_set_op_attrs(bio, REQ_OP_WRITE, flags);
iomap_dio_submit_bio(dio, iomap, bio, pos);
--
2.28.0