[PATCH 0/2] fuse: remove extra page copies in writeback

[Joanne Koong <joannelkoong@gmail.com>]

Currently when we handle writeback in fuse, we allocate and copy data
to a temporary folio in order to mitigate the following deadlock scenario
that may arise if reclaim waits on writeback to complete:
* single-threaded FUSE server is in the middle of handling a request
  that needs a memory allocation
* memory allocation triggers direct reclaim
* direct reclaim waits on a folio under writeback
* the FUSE server can't write back the folio since it's stuck in
  direct reclaim
(more details can be found here [1])

The first patch adds an ASOP_NO_RECLAIM_IN_WRITEBACK flag to the struct
address_space_operations to indicate that reclaim when in writeback should
be skipped for filesystems with this flag set. More details can be found in
the commit message of the first patch, but this only actually affects the
case of legacy cgroupv1 when it encounters a folio that already has the
reclaim flag set (in the other cases, we already skip reclaim when in
writeback). Doing so allows us to get rid of needing to allocate and copy over
pages to a temporary folio when handling writeback in fuse (2nd patch).
Benchmarks can be found in commit message of the 2nd patch.

Thanks,
Joanne

[1] https://lore.kernel.org/linux-kernel/495d2400-1d96-4924-99d3-8b2952e05fc3@linux.alibaba.com/

Joanne Koong (2):
  mm: skip reclaiming folios in writeback contexts that may trigger
    deadlock
  fuse: remove tmp folio for writebacks and internal rb tree

 fs/fuse/file.c     | 321 ++++-----------------------------------------
 include/linux/fs.h |  14 ++
 mm/vmscan.c        |   6 +-
 3 files changed, 45 insertions(+), 296 deletions(-)

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2.43.5