fiemap: single-ioctl rescan in fiemap_scan_extent - #14
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fiemap_scan_extent() only needs the first extent's physical offset, but went through do_fiemap_range(), which issues a count ioctl, a calloc, and a second ioctl to map the whole range. Add fiemap_first_extent_poff(): a single fiemap ioctl into a one-extent stack buffer, no count pass, no allocation. Follow-up to the ranged-fiemap change (port of markfasheh#402). On an extent-dedupe run over a ~400-extent shared region, FIEMAP ioctls during dedupe dropped from 1336 to 660 (~2x on that path). Data is preserved (file digests unchanged) and sharing is unaffected; 39 integration tests pass. Co-Authored-By: Claude Fable 5 <[email protected]>
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Follow-up to the markfasheh#402 port.
fiemap_scan_extent()only reads the first extent's physical offset, but went throughdo_fiemap_range()— which does a count ioctl, acalloc, then a second ioctl to map the whole range.Adds
fiemap_first_extent_poff(): a singleFS_IOC_FIEMAPioctl into a one-extent stack buffer (no count pass, no allocation), and uses it infiemap_scan_extent(). Keeps the ioctl infiemap.cbehind a documented helper.Measured on an extent-dedupe run over a ~400-extent shared region: FIEMAP ioctls during dedupe 1336 → 660 (~2×) on that path. Data preserved (file digests unchanged before/after), sharing unaffected, 39 integration tests pass.
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