Re: [PATCH 7/9] iomap: write iomap validity checks

Darrick J. Wong
2022-11-17T18:40:05Z

On Thu, Nov 17, 2022 at 04:58:08PM +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.
>
> The iomap needs to grow some storage for the validity cookie that
> the filesystem provides to travel with the iomap. XFS, in
> particular, also needs to know some more information about what the
> iomap maps (attribute extents rather than file data extents) to for
> the validity cookie to cover all the types of iomaps we might need
> to validate.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

OK, I think grok this enough to:
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> fs/iomap/buffered-io.c | 29 +++++++++++++++++++++++++++-
> fs/iomap/iter.c | 19 ++++++++++++++++++-
> include/linux/iomap.h | 43 ++++++++++++++++++++++++++++++++++--------
> 3 files changed, 81 insertions(+), 10 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 028cdf115477..1a7702de76fb 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -584,7 +584,7 @@ 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,
> +static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
> size_t len, struct folio **foliop)
> {
> const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
> @@ -618,6 +618,27 @@ 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 (page_ops && page_ops->iomap_valid) {
> + bool iomap_valid = page_ops->iomap_valid(iter->inode,
> + &iter->iomap);
> + if (!iomap_valid) {
> + iter->iomap.flags |= IOMAP_F_STALE;
> + status = 0;
> + goto out_unlock;
> + }
> + }
> +
> if (pos + len > folio_pos(folio) + folio_size(folio))
> len = folio_pos(folio) + folio_size(folio) - pos;
>
> @@ -773,6 +794,8 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> status = iomap_write_begin(iter, 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))
> @@ -1067,6 +1090,8 @@ static loff_t iomap_unshare_iter(struct iomap_iter *iter)
> status = iomap_write_begin(iter, 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))
> @@ -1122,6 +1147,8 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
> status = iomap_write_begin(iter, 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)
> 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 6bbed915c83a..2ecdd9d90efc 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -49,26 +49,35 @@ struct vm_fault;
> *
> * IOMAP_F_BUFFER_HEAD indicates that the file system requires the use of
> * buffer heads for this mapping.
> + *
> + * IOMAP_F_XATTR indicates that the iomap is for an extended attribute extent
> + * rather than a file data extent.
> */
> -#define IOMAP_F_NEW 0x01
> -#define IOMAP_F_DIRTY 0x02
> -#define IOMAP_F_SHARED 0x04
> -#define IOMAP_F_MERGED 0x08
> -#define IOMAP_F_BUFFER_HEAD 0x10
> -#define IOMAP_F_ZONE_APPEND 0x20
> +#define IOMAP_F_NEW (1U << 0)
> +#define IOMAP_F_DIRTY (1U << 1)
> +#define IOMAP_F_SHARED (1U << 2)
> +#define IOMAP_F_MERGED (1U << 3)
> +#define IOMAP_F_BUFFER_HEAD (1U << 4)
> +#define IOMAP_F_ZONE_APPEND (1U << 5)
> +#define IOMAP_F_XATTR (1U << 6)
>
> /*
> * Flags set by the core iomap code during operations:
> *
> * 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_SIZE_CHANGED (1U << 8)
> +#define IOMAP_F_STALE (1U << 9)
>
> /*
> * Flags from 0x1000 up are for file system specific usage:
> */
> -#define IOMAP_F_PRIVATE 0x1000
> +#define IOMAP_F_PRIVATE (1U << 12)
>
>
> /*
> @@ -89,6 +98,7 @@ struct iomap {
> void *inline_data;
> void *private; /* filesystem private */
> const struct iomap_page_ops *page_ops;
> + u64 validity_cookie; /* used with .iomap_valid() */
> };
>
> static inline sector_t iomap_sector(const struct iomap *iomap, loff_t pos)
> @@ -128,6 +138,23 @@ struct iomap_page_ops {
> int (*page_prepare)(struct inode *inode, loff_t pos, unsigned len);
> void (*page_done)(struct inode *inode, loff_t pos, unsigned copied,
> struct page *page);
> +
> + /*
> + * 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.
> + *
> + * The filesystem can store internal state (e.g. a sequence number) in
> + * iomap->validity_cookie when the iomap is first mapped to be able to
> + * detect changes between mapping time and whenever .iomap_valid() is
> + * called.
> + *
> + * 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);
> };
>
> /*
> --
> 2.37.2
>