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50d7b4332f
Patch series "mm: memfd_luo: fixes for folio flag preservation".
This series contains a couple fixes for flag preservation for memfd live
update.
The first patch fixes memfd preservation when fallocate() was used to
pre-allocate some pages. For these memfds, all the writes to fallocated
pages touched after preserve were lost.
The second patch fixes dirty flag tracking. If the dirty flag is not
tracked correctly, the next kernel might incorrectly reclaim some folios
under memory pressure, losing user data. This is a theoretical bug that I
observed when reading the code, and haven't been able to reproduce it.
This patch (of 2):
When a folio is added to a shmem file via fallocate, it is not zeroed on
allocation. This is done as a performance optimization since it is
possible the folio will never end up being used at all. When the folio is
used, shmem checks for the uptodate flag, and if absent, zeroes the folio
(and sets the flag) before returning to user.
With LUO, the flags of each folio are saved at preserve time. It is
possible to have a memfd with some folios fallocated but not uptodate.
For those, the uptodate flag doesn't get saved. The folios might later
end up being used and become uptodate. They would get passed to the next
kernel via KHO correctly since they did get preserved. But they won't
have the MEMFD_LUO_FOLIO_UPTODATE flag.
This means that when the memfd is retrieved, the folios will be added to
the shmem file without the uptodate flag. They will be zeroed before
first use, losing the data in those folios.
Since we take a big performance hit in allocating, zeroing, and pinning
all folios at prepare time anyway, take some more and zero all
non-uptodate ones too.
Later when there is a stronger need to make prepare faster, this can be
optimized.
To avoid racing with another uptodate operation, take the folio lock.
Link: https://lkml.kernel.org/r/20260223173931.2221759-2-pratyush@kernel.org
Fixes:
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