Re: [PATCH 5/7] iomap: write iomap validity checks

Darrick J. Wong
2022-11-02T16:57:55Z

On Tue, Nov 01, 2022 at 11:34:10AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> A recent multithreaded write data corruption has been uncovered in
> the iomap write code. The core of the problem is partial folio
> writes can be flushed to disk while a new racing write can map it
> and fill the rest of the page:
>
> writeback new write
>
> allocate blocks
> blocks are unwritten
> submit IO
> .....
> map blocks
> iomap indicates UNWRITTEN range
> loop {
> lock folio
> copyin data
> .....
> IO completes
> runs unwritten extent conv
> blocks are marked written
> <iomap now stale>
> get next folio
> }
>
> Now add memory pressure such that memory reclaim evicts the
> partially written folio that has already been written to disk.
>
> When the new write finally gets to the last partial page of the new
> write, it does not find it in cache, so it instantiates a new page,
> sees the iomap is unwritten, and zeros the part of the page that
> it does not have data from. This overwrites the data on disk that
> was originally written.
>
> The full description of the corruption mechanism can be found here:
>
> https://lore.kernel.org/linux-xfs/20220817093627.GZ3600936@dread.disaster.area/
>
> To solve this problem, we need to check whether the iomap is still
> valid after we lock each folio during the write. We have to do it
> after we lock the page so that we don't end up with state changes
> occurring while we wait for the folio to be locked.
>
> Hence we need a mechanism to be able to check that the cached iomap
> is still valid (similar to what we already do in buffered
> writeback), and we need a way for ->begin_write to back out and
> tell the high level iomap iterator that we need to remap the
> remaining write range.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/iomap/buffered-io.c | 53 +++++++++++++++++++++++++++++++++---------
> fs/iomap/iter.c | 19 ++++++++++++++-
> include/linux/iomap.h | 17 ++++++++++++++
> 3 files changed, 77 insertions(+), 12 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 91ee0b308e13..d3c565aa29f8 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -584,8 +584,9 @@ static int iomap_write_begin_inline(const struct iomap_iter *iter,
> return iomap_read_inline_data(iter, folio);
> }
>
> -static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
> - size_t len, struct folio **foliop)
> +static int iomap_write_begin(struct iomap_iter *iter,
> + const struct iomap_ops *ops, loff_t pos, size_t len,

I wonder if iomap_iter() should simply stuff its ops pointer into struct
iomap_iter so that we don't have to pass around so many arguments? Eh,
whatever, minor quibble for something that (so far) only applies to the
buffered io path.

> + struct folio **foliop)
> {
> const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
> const struct iomap *srcmap = iomap_iter_srcmap(iter);
> @@ -618,6 +619,27 @@ static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
> status = (iter->flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOMEM;
> goto out_no_page;
> }
> +
> + /*
> + * Now we have a locked folio, before we do anything with it we need to
> + * check that the iomap we have cached is not stale. The inode extent
> + * mapping can change due to concurrent IO in flight (e.g.
> + * IOMAP_UNWRITTEN state can change and memory reclaim could have
> + * reclaimed a previously partially written page at this index after IO
> + * completion before this write reaches this file offset) and hence we
> + * could do the wrong thing here (zero a page range incorrectly or fail
> + * to zero) and corrupt data.
> + */
> + if (ops->iomap_valid) {
> + bool iomap_valid = ops->iomap_valid(iter->inode, &iter->iomap);
> +
> + if (!iomap_valid) {
> + iter->iomap.flags |= IOMAP_F_STALE;
> + status = 0;
> + goto out_unlock;
> + }
> + }

The basic premise looks sound, but I wonder if a more efficient way to
do this would be to add a u32 sequence counter and a u32 pointer to
struct iomap and perform the comparison directly?

I'm saying that purely from a post-spectre "minimize indirect calls in
hot paths" perspective. From a design perspective the way you've
structured this has a (IMHO) lower coupling factor.

Anyway, that's an optimization that can happen later.

> +
> if (pos + len > folio_pos(folio) + folio_size(folio))
> len = folio_pos(folio) + folio_size(folio) - pos;
>
> @@ -727,7 +749,8 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
> return ret;
> }
>
> -static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> +static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i,
> + const struct iomap_ops *ops)
> {
> loff_t length = iomap_length(iter);
> loff_t pos = iter->pos;
> @@ -770,9 +793,11 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> break;
> }
>
> - status = iomap_write_begin(iter, pos, bytes, &folio);
> + status = iomap_write_begin(iter, ops, pos, bytes, &folio);
> if (unlikely(status))
> break;
> + if (iter->iomap.flags & IOMAP_F_STALE)
> + break;
>
> page = folio_file_page(folio, pos >> PAGE_SHIFT);
> if (mapping_writably_mapped(mapping))
> @@ -825,14 +850,15 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
> iter.flags |= IOMAP_NOWAIT;
>
> while ((ret = iomap_iter(&iter, ops)) > 0)
> - iter.processed = iomap_write_iter(&iter, i);
> + iter.processed = iomap_write_iter(&iter, i, ops);
> if (iter.pos == iocb->ki_pos)
> return ret;
> return iter.pos - iocb->ki_pos;
> }
> EXPORT_SYMBOL_GPL(iomap_file_buffered_write);
>
> -static loff_t iomap_unshare_iter(struct iomap_iter *iter)
> +static loff_t iomap_unshare_iter(struct iomap_iter *iter,
> + const struct iomap_ops *ops)
> {
> struct iomap *iomap = &iter->iomap;
> const struct iomap *srcmap = iomap_iter_srcmap(iter);
> @@ -853,9 +879,11 @@ static loff_t iomap_unshare_iter(struct iomap_iter *iter)
> unsigned long bytes = min_t(loff_t, PAGE_SIZE - offset, length);
> struct folio *folio;
>
> - status = iomap_write_begin(iter, pos, bytes, &folio);
> + status = iomap_write_begin(iter, ops, pos, bytes, &folio);
> if (unlikely(status))
> return status;
> + if (iter->iomap.flags & IOMAP_F_STALE)
> + break;
>
> status = iomap_write_end(iter, pos, bytes, bytes, folio);
> if (WARN_ON_ONCE(status == 0))
> @@ -886,12 +914,13 @@ iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
> int ret;
>
> while ((ret = iomap_iter(&iter, ops)) > 0)
> - iter.processed = iomap_unshare_iter(&iter);
> + iter.processed = iomap_unshare_iter(&iter, ops);
> return ret;
> }
> EXPORT_SYMBOL_GPL(iomap_file_unshare);
>
> -static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
> +static loff_t iomap_zero_iter(struct iomap_iter *iter,
> + const struct iomap_ops *ops, bool *did_zero)
> {
> const struct iomap *srcmap = iomap_iter_srcmap(iter);
> loff_t pos = iter->pos;
> @@ -908,9 +937,11 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
> size_t offset;
> size_t bytes = min_t(u64, SIZE_MAX, length);
>
> - status = iomap_write_begin(iter, pos, bytes, &folio);
> + status = iomap_write_begin(iter, ops, pos, bytes, &folio);
> if (status)
> return status;
> + if (iter->iomap.flags & IOMAP_F_STALE)
> + break;
>
> offset = offset_in_folio(folio, pos);
> if (bytes > folio_size(folio) - offset)
> @@ -946,7 +977,7 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
> int ret;
>
> while ((ret = iomap_iter(&iter, ops)) > 0)
> - iter.processed = iomap_zero_iter(&iter, did_zero);
> + iter.processed = iomap_zero_iter(&iter, ops, did_zero);
> return ret;
> }
> EXPORT_SYMBOL_GPL(iomap_zero_range);
> diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> index a1c7592d2ade..79a0614eaab7 100644
> --- a/fs/iomap/iter.c
> +++ b/fs/iomap/iter.c
> @@ -7,12 +7,28 @@
> #include <linux/iomap.h>
> #include "trace.h"
>
> +/*
> + * Advance to the next range we need to map.
> + *
> + * If the iomap is marked IOMAP_F_STALE, it means the existing map was not fully
> + * processed - it was aborted because the extent the iomap spanned may have been
> + * changed during the operation. In this case, the iteration behaviour is to
> + * remap the unprocessed range of the iter, and that means we may need to remap
> + * even when we've made no progress (i.e. iter->processed = 0). Hence the
> + * "finished iterating" case needs to distinguish between
> + * (processed = 0) meaning we are done and (processed = 0 && stale) meaning we
> + * need to remap the entire remaining range.
> + */
> static inline int iomap_iter_advance(struct iomap_iter *iter)
> {
> + bool stale = iter->iomap.flags & IOMAP_F_STALE;
> +
> /* handle the previous iteration (if any) */
> if (iter->iomap.length) {
> - if (iter->processed <= 0)
> + if (iter->processed < 0)
> return iter->processed;
> + if (!iter->processed && !stale)
> + return 0;
> if (WARN_ON_ONCE(iter->processed > iomap_length(iter)))
> return -EIO;
> iter->pos += iter->processed;
> @@ -33,6 +49,7 @@ static inline void iomap_iter_done(struct iomap_iter *iter)
> WARN_ON_ONCE(iter->iomap.offset > iter->pos);
> WARN_ON_ONCE(iter->iomap.length == 0);
> WARN_ON_ONCE(iter->iomap.offset + iter->iomap.length <= iter->pos);
> + WARN_ON_ONCE(iter->iomap.flags & IOMAP_F_STALE);
>
> trace_iomap_iter_dstmap(iter->inode, &iter->iomap);
> if (iter->srcmap.type != IOMAP_HOLE)
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 238a03087e17..308931f0840a 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -62,8 +62,13 @@ struct vm_fault;
> *
> * IOMAP_F_SIZE_CHANGED indicates to the iomap_end method that the file size
> * has changed as the result of this write operation.
> + *
> + * IOMAP_F_STALE indicates that the iomap is not valid any longer and the file
> + * range it covers needs to be remapped by the high level before the operation
> + * can proceed.
> */
> #define IOMAP_F_SIZE_CHANGED 0x100
> +#define IOMAP_F_STALE 0x200

Nit: might it be time to convert these to "(1U << NN)" format?

> /*
> * Flags from 0x1000 up are for file system specific usage:
> @@ -165,6 +170,18 @@ struct iomap_ops {
> */
> int (*iomap_end)(struct inode *inode, loff_t pos, loff_t length,
> ssize_t written, unsigned flags, struct iomap *iomap);
> +
> + /*
> + * Check that the cached iomap still maps correctly to the filesystem's
> + * internal extent map. FS internal extent maps can change while iomap
> + * is iterating a cached iomap, so this hook allows iomap to detect that
> + * the iomap needs to be refreshed during a long running write
> + * operation.
> + *
> + * This is called with the folio over the specified file position
> + * held locked by the iomap code.
> + */
> + bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);

Hmmm so now that I remembered that iomap_page_ops are passed back via
struct iomap, now I'm drifting towards agreeing with hch that the
function pointer should be there, not iomap_ops.

(The mechanics of this patch look correct to me, fwiw.)

--D

> };
>
> /**
> --
> 2.37.2
>