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f83f362d40
In dirty_ratio_handler(), vm_dirty_bytes must be set to zero before
calling writeback_set_ratelimit(), as global_dirty_limits() always
prioritizes the value of vm_dirty_bytes.
It's domain_dirty_limits() that's relevant here, not node_dirty_ok:
dirty_ratio_handler
writeback_set_ratelimit
global_dirty_limits(&dirty_thresh) <- ratelimit_pages based on dirty_thresh
domain_dirty_limits
if (bytes) <- bytes = vm_dirty_bytes <--------+
thresh = f1(bytes) <- prioritizes vm_dirty_bytes |
else |
thresh = f2(ratio) |
ratelimit_pages = f3(dirty_thresh) |
vm_dirty_bytes = 0 <- it's late! ---------------------+
This causes ratelimit_pages to still use the value calculated based on
vm_dirty_bytes, which is wrong now.
The impact visible to userspace is difficult to capture directly because
there is no procfs/sysfs interface exported to user space. However, it
will have a real impact on the balance of dirty pages.
For example:
1. On default, we have vm_dirty_ratio=40, vm_dirty_bytes=0
2. echo 8192 > dirty_bytes, then vm_dirty_bytes=8192,
vm_dirty_ratio=0, and ratelimit_pages is calculated based on
vm_dirty_bytes now.
3. echo 20 > dirty_ratio, then since vm_dirty_bytes is not reset to
zero when writeback_set_ratelimit() -> global_dirty_limits() ->
domain_dirty_limits() is called, reallimit_pages is still calculated
based on vm_dirty_bytes instead of vm_dirty_ratio. This does not
conform to the actual intent of the user.
Link: https://lkml.kernel.org/r/20250415090232.7544-1-alexjlzheng@tencent.com
Fixes:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.