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When a file mapping covers a strict subset of a file, an access to the
mapping can trigger readahead of file pages outside the mapped region.
Readahead is meant to prefetch pages likely to be accessed soon, but these
pages aren't accessible via the same means, so it fair to say we don't
have a good indicator they'll be accessed soon. Take an ELF file for
example: an access to the end of a program's read-only segment isn't a
sign that nearby file contents will be accessed next (they are likely to
be mapped discontiguously, or not at all). The pressure from loading
these pages into the cache can evict more useful pages.
To improve the behavior, make three changes:
* Introduce a new readahead_control field, max_index, as a hard limit on
the readahead. The existing file_ra_state->size can't be used as a
limit, it is more of a hint and can be increased by various
heuristics.
* Set readahead_control->max_index to the end of the VMA in all of the
readahead paths that can be triggered from a fault on a file mapping
(both "sync" and "async" readahead).
* Limit the read-around range start to the VMA's start.
Note that these changes only affect readahead triggered in the context of
a fault, they do not affect readahead triggered by read syscalls. If a
user mixes the two types of accesses, the behavior is expected to be the
following: if a fault causes readahead and places a PG_readahead marker
and then a read(2) syscall hits the PG_readahead marker, the resulting
async readahead *will not* be limited to the VMA end. Conversely, if a
read(2) syscall places a PG_readahead marker and then a fault hits the
marker, the async readahead *will* be limited to the VMA end.
There is an edge case that the above motivation glosses over: A single
file mapping might be backed by multiple VMAs. For example, a whole file
could be mapped RW, then part of the mapping made RO using mprotect. This
patch would hurt performance of a sequential faulted read of such a
mapping, the degree depending on how fragmented the VMAs are. A usage
pattern like that is likely rare and already suffering from sub-optimal
performance because, e.g., the fragmented VMAs limit the fault-around, so
each VMA boundary in a sequential faulted read would cause a minor fault.
Still, this patch would make it worse. See a previous discussion of this
topic at [1].
Tested by mapping and reading a small subset of a large file, then using
the cachestat syscall to verify the number of cached pages didn't exceed
the mapping size.
In practical scenarios, the effect depends on the specific file and usage.
Sometimes there is no effect at all, but, for some ELF files in Android,
we see ~20% fewer pages pulled into the cache.
A comprehensive performance evaluation hasn't been done, but, in addition
to the anecdontal memory savings mentioned above, a benchmark was run with
fio 3.38, showing neutral looking results:
/data/local/tmp/fio --version
fio --name=mmap_test --ioengine=mmap --rw=read --bs=4k \
--offset=1G --size=1G --filesize=3G --numjobs=1 \
--filename=testfile.bin
Before: 4366.6 MiB/s (avg of 3459, 4592, 4613, 4697, 4472)
After: 4444.0 MiB/s (avg of 4633, 4655, 4511, 4571, 3850)
+1.7%
Same, with --ioengine=mmap --rw=randread
Before: 445.6 MiB/s (avg of 446, 447, 442, 452, 441)
After: 447.0 MiB/s (avg of 447, 446, 446, 451, 445)
+0.3%
Same, with --ioengine=psync --rw=read
Before: 3086.6 MiB/s (avg of 3122, 3094, 3066, 3094, 3057)
After: 3084.6 MiB/s (avg of 3039, 3103, 3103, 3084, 3094)
-0.06%
Same, with --ioengine=psync --rw=randread
Before: 2226.4 MiB/s (avg of 2256, 2183, 2207, 2265, 2221)
After: 2231.4 MiB/s (avg of 2236, 2241, 2236, 2193, 2251)
+0.2%
Link: https://lore.kernel.org/20260427030148.653228-1-fmayle@google.com
Link: https://lore.kernel.org/all/ivnv2crd3et76p2nx7oszuqhzzah756oecn5yuykzqfkqzoygw@yvnlkhjjssoz/ [1]
Signed-off-by: Frederick Mayle <fmayle@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kalesh Singh <kaleshsingh@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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| kernel | ||
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| COPYING | ||
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| MAINTAINERS | ||
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| README | ||
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst