[RFC PATCH 0/3] Neural Storage Driver - learning page cache prefetcher

[Ayhan Aydin <nsd.project.dev@gmail.com>]

Hello,

This RFC introduces NSD (Neural Storage Driver), a learning prefetcher
for the Linux kernel page cache. NSD monitors I/O patterns via a hook
in filemap_read() and prefetches pages using page_cache_sync_readahead().

Background
==========
The kernel existing readahead uses a fixed-window approach that works
well for purely sequential access. NSD builds on this by learning
access patterns at 4KB region granularity using a synaptic Markov chain:

  - Records transitions between file regions
  - Detects sequential strides of arbitrary length
  - Issues page_cache_sync_readahead() for predicted pages
  - Achieves 98% real hit rate on prefetched pages (SSD)

Performance (x86_64, SATA SSD, kernel 7.0.0)
=============================================
  SQLite FTS (4GB table):  -18.8% query time
  Sequential 64K buffered: +22.6% throughput
  Random 4K buffered:      +1.1% (noise)

Read latency:              130us (no regression)

Key Question
============
Is this approach suitable for the Linux readahead infrastructure?
I would like to hear your opinions on the architecture.

The module currently hooks filemap_read() and issues prefetch via
page_cache_sync_readahead(). Would a deeper integration with the
existing ra_state / file_ra_struct be preferred? Should we extend
file_ra_struct to accommodate pattern history, or keep the predictor
as a separate subsystem?

Patch Summary
=============
  1/3: mm/filemap: Add NSD prefetch hook point (+5 lines)
  2/3: nsd: Core prediction engine (fs/nsd/, ~480 lines)
  3/3: Documentation: Add NSD documentation and MAINTAINERS entry

Signed-off-by: Ayhan Aydin <nsd.project.dev@gmail.com>
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