Re: [PATCH v5 3/7] fs: iomap: Atomic write support

From: John Garry
Date: Thu Aug 22 2024 - 11:34:25 EST

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In reply to: Darrick J. Wong: "Re: [PATCH v5 3/7] fs: iomap: Atomic write support"
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+
static void iomap_dio_submit_bio(const struct iomap_iter *iter,
struct iomap_dio *dio, struct bio *bio, loff_t pos)
{
@@ -256,7 +275,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
* clearing the WRITE_THROUGH flag in the dio request.
*/
static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
- const struct iomap *iomap, bool use_fua)
+ const struct iomap *iomap, bool use_fua, bool atomic)
{
blk_opf_t opflags = REQ_SYNC | REQ_IDLE;
@@ -268,6 +287,8 @@ static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
opflags |= REQ_FUA;
else
dio->flags &= ~IOMAP_DIO_WRITE_THROUGH;
+ if (atomic)
+ opflags |= REQ_ATOMIC;
return opflags;
}
@@ -275,21 +296,23 @@ static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
struct iomap_dio *dio)
{
+ bool atomic = dio->iocb->ki_flags & IOCB_ATOMIC;
const struct iomap *iomap = &iter->iomap;
struct inode *inode = iter->inode;
unsigned int fs_block_size = i_blocksize(inode), pad;
+ struct iov_iter *i = dio->submit.iter;

If you're going to pull this out into a convenience variable, please do
that as a separate patch so that the actual untorn write additions don't
get mixed in.

Yeah, I was thinking of doing that, so ok.

loff_t length = iomap_length(iter);
loff_t pos = iter->pos;
blk_opf_t bio_opf;
struct bio *bio;
bool need_zeroout = false;
bool use_fua = false;
- int nr_pages, ret = 0;
+ int nr_pages, orig_nr_pages, ret = 0;
size_t copied = 0;
size_t orig_count;
if ((pos | length) & (bdev_logical_block_size(iomap->bdev) - 1) ||
- !bdev_iter_is_aligned(iomap->bdev, dio->submit.iter))
+ !bdev_iter_is_aligned(iomap->bdev, i))
return -EINVAL;
if (iomap->type == IOMAP_UNWRITTEN) {
@@ -322,15 +345,35 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
dio->flags &= ~IOMAP_DIO_CALLER_COMP;
}
+ if (dio->atomic_bio) {
+ /*
+ * These should not fail, but check just in case.
+ * Caller takes care of freeing the bio.
+ */
+ if (iter->iomap.bdev != dio->atomic_bio->bi_bdev) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ if (dio->atomic_bio->bi_iter.bi_sector +
+ (dio->atomic_bio->bi_iter.bi_size >> SECTOR_SHIFT) !=

Hmm, so I guess you stash an untorn write bio in the iomap_dio so that
multiple iomap_dio_bio_iter can try to combine a mixed mapping into a
single contiguous untorn write that can be completed all at once?

Right, we are writing to a contiguous LBA address range with a single bio that happens to cover many different extents.

I suppose that works as long as the iomap->type is the same across all
the _iter calls, but I think that needs explicit checking here.

As an sample, if we try to atomically write over the data in the following file:

# xfs_bmap -vvp mnt/file
mnt/file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..127]: hole 128
1: [128..135]: 256..263 0 (256..263) 8 010000
2: [136..143]: 264..271 0 (264..271) 8 000000
3: [144..255]: 272..383 0 (272..383) 112 010000
FLAG Values:
0100000 Shared extent
0010000 Unwritten preallocated extent
0001000 Doesn't begin on stripe unit
0000100 Doesn't end on stripe unit
0000010 Doesn't begin on stripe width
0000001 Doesn't end on stripe width
#

Then, the iomap->type/flag is either IOMAP_UNWRITTEN/IOMAP_F_DIRTY or IOMAP_MAPPED/IOMAP_F_DIRTY per iter. So the type is not consistent. However we will set IOMAP_DIO_UNWRITTEN in dio->flags, so call xfs_dio_write_endio() -> xfs_iomap_write_unwritten() for the complete FSB range.

Do you see a problem with this?

Please see this also for some more background:
https://lore.kernel.org/linux-xfs/20240726171358.GA27612@xxxxxx/

+ iomap_sector(iomap, pos)) {
+ ret = -EINVAL;
+ goto out;
+ }
+ } else if (atomic) {
+ orig_nr_pages = bio_iov_vecs_to_alloc(i, BIO_MAX_VECS);
+ }
+
/*
* Save the original count and trim the iter to just the extent we
* are operating on right now. The iter will be re-expanded once
* we are done.
*/
- orig_count = iov_iter_count(dio->submit.iter);
- iov_iter_truncate(dio->submit.iter, length);
+ orig_count = iov_iter_count(i);
+ iov_iter_truncate(i, length);
- if (!iov_iter_count(dio->submit.iter))
+ if (!iov_iter_count(i))
goto out;
/*
@@ -365,27 +408,46 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
* can set up the page vector appropriately for a ZONE_APPEND
* operation.
*/
- bio_opf = iomap_dio_bio_opflags(dio, iomap, use_fua);
+ bio_opf = iomap_dio_bio_opflags(dio, iomap, use_fua, atomic);
+
+ if (atomic) {
+ size_t orig_atomic_size;
+
+ if (!dio->atomic_bio) {
+ dio->atomic_bio = iomap_dio_alloc_bio_data(iter,
+ dio, orig_nr_pages, bio_opf, pos);
+ }
+ orig_atomic_size = dio->atomic_bio->bi_iter.bi_size;
+
+ /*
+ * In case of error, caller takes care of freeing the bio. The
+ * smallest size of atomic write is i_node size, so no need for

What is "i_node size"? Are you referring to i_blocksize?

Yes, I meant i_blocksize()

+ * tail zeroing out.
+ */
+ ret = bio_iov_iter_get_pages(dio->atomic_bio, i);
+ if (!ret) {
+ copied = dio->atomic_bio->bi_iter.bi_size -
+ orig_atomic_size;
+ }
- nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_VECS);
+ dio->size += copied;
+ goto out;
+ }
+
+ nr_pages = bio_iov_vecs_to_alloc(i, BIO_MAX_VECS);
do {
size_t n;
if (dio->error) {
- iov_iter_revert(dio->submit.iter, copied);
+ iov_iter_revert(i, copied);
copied = ret = 0;
goto out;
}
- bio = iomap_dio_alloc_bio(iter, dio, nr_pages, bio_opf);
+ bio = iomap_dio_alloc_bio_data(iter, dio, nr_pages, bio_opf, pos);
fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
GFP_KERNEL);
- bio->bi_iter.bi_sector = iomap_sector(iomap, pos);
- bio->bi_write_hint = inode->i_write_hint;
- bio->bi_ioprio = dio->iocb->ki_ioprio;
- bio->bi_private = dio;
- bio->bi_end_io = iomap_dio_bio_end_io;

I see two places (here and iomap_dio_zero) that allocate a bio and
perform some initialization of it. Can you move the common pieces to
iomap_dio_alloc_bio instead of adding a iomap_dio_alloc_bio_data
variant, and move all that to a separate cleanup patch?

Sure

So can it cause harm if we set bio->bi_write_hint and ->bi_ioprio with the same values as iomap_dio_alloc_bio() for iomap_dio_zero()? If no, this would help make all the bio alloc code common

- ret = bio_iov_iter_get_pages(bio, dio->submit.iter);
+ ret = bio_iov_iter_get_pages(bio, i);
if (unlikely(ret)) {
/*
* We have to stop part way through an IO. We must fall
@@ -408,8 +470,7 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
dio->size += n;
copied += n;
- nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter,
- BIO_MAX_VECS);
+ nr_pages = bio_iov_vecs_to_alloc(i, BIO_MAX_VECS);
/*
* We can only poll for single bio I/Os.
*/
@@ -435,7 +496,7 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
}
out:
/* Undo iter limitation to current extent */
- iov_iter_reexpand(dio->submit.iter, orig_count - copied);
+ iov_iter_reexpand(i, orig_count - copied);
if (copied)
return copied;
return ret;
@@ -555,6 +616,7 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
struct blk_plug plug;
struct iomap_dio *dio;
loff_t ret = 0;
+ size_t orig_count = iov_iter_count(iter);
trace_iomap_dio_rw_begin(iocb, iter, dio_flags, done_before);
@@ -580,6 +642,13 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
if (iocb->ki_flags & IOCB_NOWAIT)
iomi.flags |= IOMAP_NOWAIT;
+ if (iocb->ki_flags & IOCB_ATOMIC) {
+ if (bio_iov_vecs_to_alloc(iter, INT_MAX) > BIO_MAX_VECS)
+ return ERR_PTR(-EINVAL);
+ iomi.flags |= IOMAP_ATOMIC;
+ }
+ dio->atomic_bio = NULL;
+
if (iov_iter_rw(iter) == READ) {
/* reads can always complete inline */
dio->flags |= IOMAP_DIO_INLINE_COMP;
@@ -665,6 +734,21 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
iocb->ki_flags &= ~IOCB_HIPRI;
}
+ if (iocb->ki_flags & IOCB_ATOMIC) {
+ if (ret >= 0) {
+ if (dio->size == orig_count) {
+ iomap_dio_submit_bio(&iomi, dio,
+ dio->atomic_bio, iocb->ki_pos);

Does this need to do task_io_account_write like regular direct writes
do?

yes, I missed that, will fix

+ } else {
+ if (dio->atomic_bio)
+ bio_put(dio->atomic_bio);
+ ret = -EINVAL;
+ }
+ } else if (dio->atomic_bio) {
+ bio_put(dio->atomic_bio);

This ought to null out dio->atomic_bio to prevent accidental UAF.

ok, fine

Thanks,
John

Next message: Mark Brown: "Re: [PATCH 0/2] KVM: arm64: Control visibility of S1PIE related sysregs to userspace"
Previous message: Claudiu: "[PATCH 14/16] arm64: dts: renesas: r9a08g045: Add USB support"
In reply to: Darrick J. Wong: "Re: [PATCH v5 3/7] fs: iomap: Atomic write support"
Next in thread: Darrick J. Wong: "Re: [PATCH v5 3/7] fs: iomap: Atomic write support"
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