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xfs: don't screw up direct writes when freesp is fragmented
xfs_bmap_btalloc is given a range of file offset blocks that must be allocated to some data/attr/cow fork. If the fork has an extent size hint associated with it, the request will be enlarged on both ends to try to satisfy the alignment hint. If free space is fragmentated, sometimes we can allocate some blocks but not enough to fulfill any of the requested range. Since bmapi_allocate always trims the new extent mapping to match the originally requested range, this results in bmapi_write returning zero and no mapping. The consequences of this vary -- buffered writes will simply re-call bmapi_write until it can satisfy at least one block from the original request. Direct IO overwrites notice nmaps == 0 and return -ENOSPC through the dio mechanism out to userspace with the weird result that writes fail even when we have enough space because the ENOSPC return overrides any partial write status. For direct CoW writes the situation was disastrous because nobody notices us returning an invalid zero-length wrong-offset mapping to iomap and the write goes off into space. Therefore, if free space is so fragmented that we managed to allocate some space but not enough to map into even a single block of the original allocation request range, we should break the alignment hint in order to guarantee at least some forward progress for the direct write. If we return a short allocation to iomap_apply it'll call back about the remaining blocks. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Reviewed-by: Christoph Hellwig <hch@lst.de>
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1 changed files with 20 additions and 0 deletions
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@ -3390,6 +3390,8 @@ xfs_bmap_btalloc(
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xfs_agnumber_t fb_agno; /* ag number of ap->firstblock */
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xfs_agnumber_t ag;
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xfs_alloc_arg_t args;
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xfs_fileoff_t orig_offset;
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xfs_extlen_t orig_length;
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xfs_extlen_t blen;
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xfs_extlen_t nextminlen = 0;
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int nullfb; /* true if ap->firstblock isn't set */
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@ -3399,6 +3401,8 @@ xfs_bmap_btalloc(
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int stripe_align;
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ASSERT(ap->length);
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orig_offset = ap->offset;
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orig_length = ap->length;
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mp = ap->ip->i_mount;
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@ -3614,6 +3618,22 @@ xfs_bmap_btalloc(
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*ap->firstblock = args.fsbno;
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ASSERT(nullfb || fb_agno <= args.agno);
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ap->length = args.len;
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/*
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* If the extent size hint is active, we tried to round the
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* caller's allocation request offset down to extsz and the
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* length up to another extsz boundary. If we found a free
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* extent we mapped it in starting at this new offset. If the
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* newly mapped space isn't long enough to cover any of the
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* range of offsets that was originally requested, move the
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* mapping up so that we can fill as much of the caller's
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* original request as possible. Free space is apparently
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* very fragmented so we're unlikely to be able to satisfy the
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* hints anyway.
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*/
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if (ap->length <= orig_length)
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ap->offset = orig_offset;
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else if (ap->offset + ap->length < orig_offset + orig_length)
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ap->offset = orig_offset + orig_length - ap->length;
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xfs_bmap_btalloc_accounting(ap, &args);
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} else {
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ap->blkno = NULLFSBLOCK;
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